Code review comment: Perhaps rather do "else", more obvious.

Code under review:
public void handle(GrpcClientRequest<Buffer, Buffer> request) {
            if (compressionEnabled) {
                request.encoding("gzip");
            }

            
            request.serviceName(ServiceName.create(GRPC_SERVICE_NAME));
            request.methodName(GRPC_METHOD_NAME);

            if (!headers.isEmpty()) {
                var vertxHeaders = request.headers();
                for (var entry : headers.entrySet()) {
                    vertxHeaders.set(entry.getKey(), entry.getValue());
                }
            }

            try {
                int messageSize = marshaler.getBinarySerializedSize();
                Buffer buffer = Buffer.buffer(messageSize);
                var os = new BufferOutputStream(buffer);
                marshaler.writeBinaryTo(os);
                request.send(buffer).onSuccess(new Handler<>() {
                    @Override
                    public void handle(GrpcClientResponse<Buffer, Buffer> response) {
                        response.exceptionHandler(new Handler<>() {
                            @Override
                            public void handle(Throwable t) {
                                exporterMetrics.addFailed(numItems);
                                logger.log(
                                        Level.SEVERE,
                                        "Failed to export "
                                                + type
                                                + "s. The response could not be parsed. Full error message: "
                                                + t.getMessage());
                                result.fail();
                            }
                        }).errorHandler(new Handler<>() {
                            @Override
                            public void handle(GrpcError error) {
                                handleError(error.status, response);
                            }
                        }).endHandler(new Handler<>() {
                            @Override
                            public void handle(Void ignored) {
                                GrpcStatus status = getStatus(response);
                                if (status == GrpcStatus.OK) {
                                    exporterMetrics.addSuccess(numItems);
                                    result.succeed();
                                    return;
                                }
                                handleError(status, response);
                            }
                        });
                    }

                    private void handleError(GrpcStatus status, GrpcClientResponse<Buffer, Buffer> response) {
                        String statusMessage = getStatusMessage(response);
                        logAppropriateWarning(status, statusMessage);
                        exporterMetrics.addFailed(numItems);
                        result.fail();
                    }

                    private void logAppropriateWarning(GrpcStatus status,
                            String statusMessage) {
                        if (status == GrpcStatus.UNIMPLEMENTED) {
                            if (loggedUnimplemented.compareAndSet(false, true)) {
                                logUnimplemented(internalLogger, type, statusMessage);
                            }
                        } else if (status == GrpcStatus.UNAVAILABLE) {
                            logger.log(
                                    Level.SEVERE,
                                    "Failed to export "
                                            + type
                                            + "s. Server is UNAVAILABLE. "
                                            + "Make sure your collector is running and reachable from this network. "
                                            + "Full error message:"
                                            + statusMessage);
                        } else {
                            if (status == null) {
                                logger.log(
                                        Level.WARNING,
                                        "Failed to export "
                                                + type
                                                + "s. Server responded with error message: "
                                                + statusMessage);
                            } else {
                                logger.log(
                                        Level.WARNING,
                                        "Failed to export "
                                                + type
                                                + "s. Server responded with "
                                                + status.code
                                                + ". Error message: "
                                                + statusMessage);
                            }
                        }
                    }

                    private void logUnimplemented(Logger logger, String type, String fullErrorMessage) {
                        String envVar;
                        switch (type) {
                            case "span":
                                envVar = "OTEL_TRACES_EXPORTER";
                                break;
                            case "metric":
                                envVar = "OTEL_METRICS_EXPORTER";
                                break;
                            case "log":
                                envVar = "OTEL_LOGS_EXPORTER";
                                break;
                            default:
                                throw new IllegalStateException(
                                        "Unrecognized type, this is a programming bug in the OpenTelemetry SDK");
                        }

                        logger.log(
                                Level.SEVERE,
                                "Failed to export "
                                        + type
                                        + "s. Server responded with UNIMPLEMENTED. "
                                        + "This usually means that your collector is not configured with an otlp "
                                        + "receiver in the \"pipelines\" section of the configuration. "
                                        + "If export is not desired and you are using OpenTelemetry autoconfiguration or the javaagent, "
                                        + "disable export by setting "
                                        + envVar
                                        + "=none. "
                                        + "Full error message: "
                                        + fullErrorMessage);
                    }

                    private GrpcStatus getStatus(GrpcClientResponse<?, ?> response) {
                        
                        GrpcStatus result = response.status();
                        if (result == null) {
                            String statusFromTrailer = response.trailers().get(GRPC_STATUS);
                            if (statusFromTrailer != null) {
                                result = GrpcStatus.valueOf(Integer.parseInt(statusFromTrailer));
                            }
                        }
                        return result;
                    }

                    private String getStatusMessage(GrpcClientResponse<Buffer, Buffer> response) {
                        
                        String result = response.statusMessage();
                        if (result == null) {
                            result = response.trailers().get(GRPC_MESSAGE);
                            if (result != null) {
                                result = QueryStringDecoder.decodeComponent(result, StandardCharsets.UTF_8);
                            }

                        }
                        return result;
                    }

                }).onFailure(new Handler<>() {
                    @Override
                    public void handle(Throwable t) {
                        exporterMetrics.addFailed(numItems);
                        logger.log(
                                Level.SEVERE,
                                "Failed to export "
                                        + type
                                        + "s. The request could not be executed. Full error message: "
                                        + t.getMessage());
                        result.fail();
                    }
                });
            } catch (IOException e) {
                exporterMetrics.addFailed(numItems);
                logger.log(
                        Level.SEVERE,
                        "Failed to export "
                                + type
                                + "s. Unable to serialize payload. Full error message: "
                                + e.getMessage());
                result.fail();
            }
        }

Line commented on: return;

Revised code:
public void handle(GrpcClientRequest<Buffer, Buffer> request) {
            if (compressionEnabled) {
                request.encoding("gzip");
            }

            
            request.serviceName(ServiceName.create(GRPC_SERVICE_NAME));
            request.methodName(GRPC_METHOD_NAME);

            if (!headers.isEmpty()) {
                var vertxHeaders = request.headers();
                for (var entry : headers.entrySet()) {
                    vertxHeaders.set(entry.getKey(), entry.getValue());
                }
            }

            try {
                int messageSize = marshaler.getBinarySerializedSize();
                Buffer buffer = Buffer.buffer(messageSize);
                var os = new BufferOutputStream(buffer);
                marshaler.writeBinaryTo(os);
                request.send(buffer).onSuccess(new Handler<>() {
                    @Override
                    public void handle(GrpcClientResponse<Buffer, Buffer> response) {
                        response.exceptionHandler(new Handler<>() {
                            @Override
                            public void handle(Throwable t) {
                                exporterMetrics.addFailed(numItems);
                                logger.log(
                                        Level.SEVERE,
                                        "Failed to export "
                                                + type
                                                + "s. The stream failed. Full error message: "
                                                + t.getMessage());
                                result.fail();
                            }
                        }).errorHandler(new Handler<>() {
                            @Override
                            public void handle(GrpcError error) {
                                handleError(error.status, response);
                            }
                        }).endHandler(new Handler<>() {
                            @Override
                            public void handle(Void ignored) {
                                GrpcStatus status = getStatus(response);
                                if (status == GrpcStatus.OK) {
                                    exporterMetrics.addSuccess(numItems);
                                    result.succeed();
                                } else {
                                    handleError(status, response);
                                }
                            }
                        });
                    }

                    private void handleError(GrpcStatus status, GrpcClientResponse<Buffer, Buffer> response) {
                        String statusMessage = getStatusMessage(response);
                        logAppropriateWarning(status, statusMessage);
                        exporterMetrics.addFailed(numItems);
                        result.fail();
                    }

                    private void logAppropriateWarning(GrpcStatus status,
                            String statusMessage) {
                        if (status == GrpcStatus.UNIMPLEMENTED) {
                            if (loggedUnimplemented.compareAndSet(false, true)) {
                                logUnimplemented(internalLogger, type, statusMessage);
                            }
                        } else if (status == GrpcStatus.UNAVAILABLE) {
                            logger.log(
                                    Level.SEVERE,
                                    "Failed to export "
                                            + type
                                            + "s. Server is UNAVAILABLE. "
                                            + "Make sure your collector is running and reachable from this network. "
                                            + "Full error message:"
                                            + statusMessage);
                        } else {
                            if (status == null) {
                                logger.log(
                                        Level.WARNING,
                                        "Failed to export "
                                                + type
                                                + "s. Server responded with error message: "
                                                + statusMessage);
                            } else {
                                logger.log(
                                        Level.WARNING,
                                        "Failed to export "
                                                + type
                                                + "s. Server responded with "
                                                + status.code
                                                + ". Error message: "
                                                + statusMessage);
                            }
                        }
                    }

                    private void logUnimplemented(Logger logger, String type, String fullErrorMessage) {
                        String envVar;
                        switch (type) {
                            case "span":
                                envVar = "OTEL_TRACES_EXPORTER";
                                break;
                            case "metric":
                                envVar = "OTEL_METRICS_EXPORTER";
                                break;
                            case "log":
                                envVar = "OTEL_LOGS_EXPORTER";
                                break;
                            default:
                                throw new IllegalStateException(
                                        "Unrecognized type, this is a programming bug in the OpenTelemetry SDK");
                        }

                        logger.log(
                                Level.SEVERE,
                                "Failed to export "
                                        + type
                                        + "s. Server responded with UNIMPLEMENTED. "
                                        + "This usually means that your collector is not configured with an otlp "
                                        + "receiver in the \"pipelines\" section of the configuration. "
                                        + "If export is not desired and you are using OpenTelemetry autoconfiguration or the javaagent, "
                                        + "disable export by setting "
                                        + envVar
                                        + "=none. "
                                        + "Full error message: "
                                        + fullErrorMessage);
                    }

                    private GrpcStatus getStatus(GrpcClientResponse<?, ?> response) {
                        
                        GrpcStatus result = response.status();
                        if (result == null) {
                            String statusFromTrailer = response.trailers().get(GRPC_STATUS);
                            if (statusFromTrailer != null) {
                                result = GrpcStatus.valueOf(Integer.parseInt(statusFromTrailer));
                            }
                        }
                        return result;
                    }

                    private String getStatusMessage(GrpcClientResponse<Buffer, Buffer> response) {
                        
                        String result = response.statusMessage();
                        if (result == null) {
                            result = response.trailers().get(GRPC_MESSAGE);
                            if (result != null) {
                                result = QueryStringDecoder.decodeComponent(result, StandardCharsets.UTF_8);
                            }

                        }
                        return result;
                    }

                }).onFailure(new Handler<>() {
                    @Override
                    public void handle(Throwable t) {
                        exporterMetrics.addFailed(numItems);
                        logger.log(
                                Level.SEVERE,
                                "Failed to export "
                                        + type
                                        + "s. The request could not be executed. Full error message: "
                                        + t.getMessage());
                        result.fail();
                    }
                });
            } catch (IOException e) {
                exporterMetrics.addFailed(numItems);
                logger.log(
                        Level.SEVERE,
                        "Failed to export "
                                + type
                                + "s. Unable to serialize payload. Full error message: "
                                + e.getMessage());
                result.fail();
            }
        }