fix(gax): record fractional latency metrics

[rahul2393]

Summary

Record GAX operation and attempt latency metrics with fractional millisecond precision instead of truncating to whole milliseconds.

Previously MetricsTracer used:

stopwatch.elapsed(TimeUnit.MILLISECONDS)

This floors sub-millisecond precision before the value is recorded into the histogram. For low-latency RPCs, this can make Cloud Monitoring percentiles look significantly lower than application-observed latency.

For example, a Spanner customer investigation showed:

• application/client observed P50: ~3.7ms
• trace-measured GAX operation latency: ~3.9ms
• exported operation_latencies P50: ~2.9ms

The apparent ~800us P50 gap looked like extra client-side latency in Spanner, but raw metric prints showed values being recorded as whole numbers only (3.0, 4.0, 7.0). The sub-ms precision was lost before histogram bucketization, so this was a rounding/truncation artifact rather than actual Spanner client overhead.

Fix

Use nanosecond elapsed time and convert to fractional milliseconds:

stopwatch.elapsed(TimeUnit.NANOSECONDS) / 1_000_000.0

Applied to:

• operation latency
• attempt latency

[gemini-code-assist]

Code Review

This pull request updates MetricsTracer to support fractional millisecond latency reporting by calculating elapsed time from nanoseconds instead of using the millisecond precision provided by the Stopwatch class. It also introduces Ticker injection to improve testability, allowing for more precise unit tests using FakeTicker. A review comment suggests replacing the magic number 1,000,000.0 with TimeUnit constants to improve code readability and self-documentation.

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sdk-platform-java/gax-java/gax/src/main/java/com/google/api/gax/tracing/MetricsTracer.java (246)

To improve readability and avoid using a magic number, consider using the TimeUnit class for the conversion. This makes the code more self-documenting about the units involved.

    return stopwatch.elapsed(TimeUnit.NANOSECONDS) / (double) TimeUnit.MILLISECONDS.toNanos(1);

[sonarqubecloud]

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

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud

[sonarqubecloud]

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

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud