Code review comment: Only targeted for tests - setting a JSON environment variable value through command-line in `live-platform-matrix.json` never seemed to work for some reason. This seemed like a simple compromise and the newly added system property is hidden. Let me know if you disagree - I can investigate how to set it as JSON directly.

Code under review:
CosmosAsyncClient buildAsyncClient(boolean logStartupInfo) {
        StopWatch stopwatch = new StopWatch();
        stopwatch.start();

        if (Configs.shouldOptInDefaultCircuitBreakerConfig()) {

            if (Configs.shouldOptInDefaultCircuitBreakerConfig()) {
                System.setProperty("COSMOS.PARTITION_LEVEL_CIRCUIT_BREAKER_CONFIG", "{\"isPartitionLevelCircuitBreakerEnabled\": true}");
            }
        }

        this.resetSessionCapturingType();
        validateConfig();
        buildConnectionPolicy();
        CosmosAsyncClient cosmosAsyncClient = new CosmosAsyncClient(this);

        if (proactiveContainerInitConfig != null) {
            cosmosAsyncClient.recordOpenConnectionsAndInitCachesStarted(proactiveContainerInitConfig.getCosmosContainerIdentities());

            Duration aggressiveWarmupDuration = proactiveContainerInitConfig
                    .getAggressiveWarmupDuration();
            if (aggressiveWarmupDuration != null) {
                cosmosAsyncClient.openConnectionsAndInitCaches(aggressiveWarmupDuration);
            } else {
                cosmosAsyncClient.openConnectionsAndInitCaches();
            }

            cosmosAsyncClient.recordOpenConnectionsAndInitCachesCompleted(proactiveContainerInitConfig.getCosmosContainerIdentities());
        } else {
            cosmosAsyncClient.recordOpenConnectionsAndInitCachesCompleted(new ArrayList<>());
        }

        if (logStartupInfo) {
            logStartupInfo(stopwatch, cosmosAsyncClient);
        }
        return cosmosAsyncClient;
    }

Line commented on: if (Configs.shouldOptInDefaultCircuitBreakerConfig()) {

Revised code:
CosmosAsyncClient buildAsyncClient(boolean logStartupInfo) {
        StopWatch stopwatch = new StopWatch();
        stopwatch.start();

        if (Configs.shouldOptInDefaultCircuitBreakerConfig()) {
            System.setProperty("COSMOS.PARTITION_LEVEL_CIRCUIT_BREAKER_CONFIG", "{\"isPartitionLevelCircuitBreakerEnabled\": true}");
        }

        this.resetSessionCapturingType();
        validateConfig();
        buildConnectionPolicy();
        CosmosAsyncClient cosmosAsyncClient = new CosmosAsyncClient(this);

        if (proactiveContainerInitConfig != null) {
            cosmosAsyncClient.recordOpenConnectionsAndInitCachesStarted(proactiveContainerInitConfig.getCosmosContainerIdentities());

            Duration aggressiveWarmupDuration = proactiveContainerInitConfig
                    .getAggressiveWarmupDuration();
            if (aggressiveWarmupDuration != null) {
                cosmosAsyncClient.openConnectionsAndInitCaches(aggressiveWarmupDuration);
            } else {
                cosmosAsyncClient.openConnectionsAndInitCaches();
            }

            cosmosAsyncClient.recordOpenConnectionsAndInitCachesCompleted(proactiveContainerInitConfig.getCosmosContainerIdentities());
        } else {
            cosmosAsyncClient.recordOpenConnectionsAndInitCachesCompleted(new ArrayList<>());
        }

        if (logStartupInfo) {
            logStartupInfo(stopwatch, cosmosAsyncClient);
        }
        return cosmosAsyncClient;
    }