feat: add Batcher#close(timeout) and Batcher#cancelOutstanding

[igorbernstein2]

There have been reports of batcher.close() hanging every once in awhile. Currently it is impossible to debug because we dont expose any internal state to analyze.

This PR adds 2 additional methods that should help in diagnosing issues:

1. close(timeout) will try to close the batcher, but if any of the underlying batch operations fail, the exception message will contain a wealth of information describing the underlying state of operations as provided by feat: add toString to futures returned by operations #3140
2. cancelOutstanding this allows for remediation for close(timeout) throwing an exception.

The intended usecase is dataflow connector's FinishBundle:

try {
  batcher.close(Duration.ofMinutes(1));
} catch(TimeoutException e) {
   // log details why the batch failed to close with the help of #3140
   logger.error(e);
   batcher.cancelOutstanding();
  batcher.close(Duration.ofMinutes(1));
}

Example exception message:

Exception in thread "main" com.google.api.gax.batching.BatchingException: Timed out trying to close batcher after PT1S. Batch request prototype: com.google.cloud.bigtable.data.v2.models.BulkMutation@2bac9ba. Outstanding batches: Batch{operation=CallbackChainRetryingFuture{super=null, latestCompletedAttemptResult=ImmediateFailedFuture@6a9d5dff[status=FAILURE, cause=[com.google.cloud.bigtable.data.v2.models.MutateRowsException: Some mutations failed to apply]], attemptResult=null, attemptSettings=TimedAttemptSettings{globalSettings=RetrySettings{totalTimeout=PT10M, initialRetryDelay=PT0.01S, retryDelayMultiplier=2.0, maxRetryDelay=PT1M, maxAttempts=0, jittered=true, initialRpcTimeout=PT1M, rpcTimeoutMultiplier=1.0, maxRpcTimeout=PT1M}, retryDelay=PT1.28S, rpcTimeout=PT1M, randomizedRetryDelay=PT0.877S, attemptCount=8, overallAttemptCount=8, firstAttemptStartTimeNanos=646922035424541}}, elements=com.google.cloud.bigtable.data.v2.models.RowMutationEntry@7a344b65}

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Fixes #<issue_number_goes_here> ☕️

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[igorbernstein2]

api compatibility is failing with:

Run REPOS_UNDER_TEST="java-datastore" ./.kokoro/presubmit/downstream-compatibility.sh
Setup maven mirror
Installing this repo's modules to local maven.
Error: 7012: com.google.api.gax.batching.Batcher: Method 'public void cancelOutstanding()' has been added to an interface
Error: 7012: com.google.api.gax.batching.Batcher: Method 'public void close(org.threeten.bp.Duration)' has been added to an interface
Error: Failed to execute goal org.codehaus.mojo:clirr-maven-plugin:2.8:check (default) on project gax: There were 2 errors. -> [Help 1]

But it is wrong because the Batcher interface is marked as @InternalExtensionOnly so adding new methods should be allowed. I tried ignoring it using a clirr ignore, but that doesnt seem to work. Whats the correct way to get it to start passing?

[igorbernstein2]

Ok, everything should be ready

[sonarcloud]

Quality Gate failed for 'gapic-generator-java-root'

Failed conditions
B Reliability Rating on New Code (required ≥ A)

See analysis details on SonarCloud

Catch issues before they fail your Quality Gate with our IDE extension SonarLint

[sonarcloud]

Quality Gate failed for 'java_showcase_integration_tests'

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)
B Reliability Rating on New Code (required ≥ A)

See analysis details on SonarCloud

Catch issues before they fail your Quality Gate with our IDE extension SonarLint