Code review comment: ~~unlikely; and sub-second interval will just overload whatever external system you have.~~

Code under review:
public MetricRegistryImpl(MetricRegistryConfiguration config, Collection<ReporterSetup> reporterConfigurations) {
		this.maximumFramesize = config.getQueryServiceMessageSizeLimit();
		this.scopeFormats = config.getScopeFormats();
		this.globalDelimiter = config.getDelimiter();
		this.terminationFuture = new CompletableFuture<>();
		this.isShutdown = false;

		
		this.reporters = new ArrayList<>(4);

		this.executor = Executors.newSingleThreadScheduledExecutor(new ExecutorThreadFactory("Flink-MetricRegistry"));

		this.queryService = null;
		this.metricQueryServiceRpcService = null;

		if (reporterConfigurations.isEmpty()) {
			
			
			LOG.info("No metrics reporter configured, no metrics will be exposed/reported.");
		} else {
			for (ReporterSetup reporterSetup : reporterConfigurations) {
				final String namedReporter = reporterSetup.getName();

				try {
					Optional<String> configuredPeriod = reporterSetup.getIntervalSettings();
					TimeUnit timeunit = TimeUnit.SECONDS;
					long period = MetricOptions.REPORTER_INTERVAL.defaultValue().getSeconds();

					if (configuredPeriod.isPresent()) {
						try {
							String[] interval = configuredPeriod.get().split(" ");
							period = Long.parseLong(interval[0]);
							timeunit = TimeUnit.valueOf(interval[1]);
						}
						catch (Exception e) {
							LOG.error("Cannot parse report interval from config: " + configuredPeriod +
									" - please use values like '10 SECONDS' or '500 MILLISECONDS'. " +
									"Using default reporting interval.");
						}
					}

					final MetricReporter reporterInstance = reporterSetup.getReporter();
					final String className = reporterInstance.getClass().getName();

					if (reporterInstance instanceof Scheduled) {
						LOG.info("Periodically reporting metrics in intervals of {} {} for reporter {} of type {}.", period, timeunit.name(), namedReporter, className);

						executor.scheduleWithFixedDelay(
								new MetricRegistryImpl.ReporterTask((Scheduled) reporterInstance), period, period, timeunit);
					} else {
						LOG.info("Reporting metrics for reporter {} of type {}.", namedReporter, className);
					}

					String delimiterForReporter = reporterSetup.getDelimiter().orElse(String.valueOf(globalDelimiter));
					if (delimiterForReporter.length() != 1) {
						LOG.warn("Failed to parse delimiter '{}' for reporter '{}', using global delimiter '{}'.", delimiterForReporter, namedReporter, globalDelimiter);
						delimiterForReporter = String.valueOf(globalDelimiter);
					}

					reporters.add(new ReporterAndSettings(
						reporterInstance,
						new ReporterScopedSettings(
							reporters.size(),
							delimiterForReporter.charAt(0),
							reporterSetup.getExcludedVariables())));
				}
				catch (Throwable t) {
					LOG.error("Could not instantiate metrics reporter {}. Metrics might not be exposed/reported.", namedReporter, t);
				}
			}
		}
	}

Line commented on: long period = MetricOptions.REPORTER_INTERVAL.defaultValue().getSeconds();

Revised code:
public MetricRegistryImpl(MetricRegistryConfiguration config, Collection<ReporterSetup> reporterConfigurations) {
		this.maximumFramesize = config.getQueryServiceMessageSizeLimit();
		this.scopeFormats = config.getScopeFormats();
		this.globalDelimiter = config.getDelimiter();
		this.terminationFuture = new CompletableFuture<>();
		this.isShutdown = false;

		
		this.reporters = new ArrayList<>(4);

		this.executor = Executors.newSingleThreadScheduledExecutor(new ExecutorThreadFactory("Flink-MetricRegistry"));

		this.queryService = null;
		this.metricQueryServiceRpcService = null;

		if (reporterConfigurations.isEmpty()) {
			
			
			LOG.info("No metrics reporter configured, no metrics will be exposed/reported.");
		} else {
			for (ReporterSetup reporterSetup : reporterConfigurations) {
				final String namedReporter = reporterSetup.getName();

				try {
					Optional<String> configuredPeriod = reporterSetup.getIntervalSettings();
					TimeUnit timeunit = TimeUnit.SECONDS;
					long period = MetricOptions.REPORTER_INTERVAL.defaultValue().getSeconds();

					if (configuredPeriod.isPresent()) {
						try {
							String[] interval = configuredPeriod.get().split(" ");
							period = Long.parseLong(interval[0]);
							timeunit = TimeUnit.valueOf(interval[1]);
						}
						catch (Exception e) {
							LOG.error("Cannot parse report interval from config: " + configuredPeriod +
									" - please use values like '10 SECONDS' or '500 MILLISECONDS'. " +
									"Using default reporting interval.");
						}
					}

					final MetricReporter reporterInstance = reporterSetup.getReporter();
					final String className = reporterInstance.getClass().getName();

					if (reporterInstance instanceof Scheduled) {
						LOG.info("Periodically reporting metrics in intervals of {} {} for reporter {} of type {}.", period, timeunit.name(), namedReporter, className);

						executor.scheduleWithFixedDelay(
								new MetricRegistryImpl.ReporterTask((Scheduled) reporterInstance), period, period, timeunit);
					} else {
						LOG.info("Reporting metrics for reporter {} of type {}.", namedReporter, className);
					}

					String delimiterForReporter = reporterSetup.getDelimiter().orElse(String.valueOf(globalDelimiter));
					if (delimiterForReporter.length() != 1) {
						LOG.warn("Failed to parse delimiter '{}' for reporter '{}', using global delimiter '{}'.", delimiterForReporter, namedReporter, globalDelimiter);
						delimiterForReporter = String.valueOf(globalDelimiter);
					}

					reporters.add(new ReporterAndSettings(
						reporterInstance,
						new ReporterScopedSettings(
							reporters.size(),
							delimiterForReporter.charAt(0),
							reporterSetup.getExcludedVariables())));
				}
				catch (Throwable t) {
					LOG.error("Could not instantiate metrics reporter {}. Metrics might not be exposed/reported.", namedReporter, t);
				}
			}
		}
	}