Record error.type on failed collections in PeriodicMetricReader

[TimurRakhmatullin86]

Problem

In PeriodicMetricReader.Scheduled.doRun() the self-observability sample for
otel.sdk.metric_reader.collection.duration is always recorded with a null error:

long startNanoTime = CLOCK.nanoTime();
String error = null;
Collection<MetricData> metricData;
try {
  metricData = collectionRegistration.collectAllMetrics();
} finally {
  long durationNanos = CLOCK.nanoTime() - startNanoTime;
  instrumentation.recordCollection(durationNanos / 1_000_000_000.0, error);
}

error is declared, initialized to null, and never assigned anywhere in the method, so
MetricReaderInstrumentation.recordCollection's if (error != null) branch that attaches the
error.type attribute is unreachable from this caller. When collectAllMetrics() throws, the
finally still records the sample — as a successful collection with no error.type — and
the exception propagates to the outer catch (Throwable). The self-observability histogram
therefore reports error-free collections while the reader is exporting nothing, which is
exactly the signal an operator would use to detect the outage.

The sibling BatchSpanProcessor.exportCurrentBatch() uses the same String error = null; … finally { instrumentation…(…, error); } shape but assigns error on each failure path.

Fix

Set error to the throwable's class name before it propagates:

try {
  metricData = collectionRegistration.collectAllMetrics();
} catch (Throwable t) {
  error = t.getClass().getName();
  throw t;
} finally {
  ...
}

The rethrow keeps the existing control flow — the exception still reaches the same outer
catch (Throwable) — so the only change is that the recorded sample now carries error.type.
(ThrowableUtil.propagateIfFatal is intentionally not added: the outer catch already handles
all throwables, so it would be a no-op here.)

Tests

SdkMeterProviderMetricsTest.collectionFailureIsRecordedWithErrorType: registers a
MetricProducer that throws on the first produce() then succeeds, flushes twice, and asserts
the exported otel.sdk.metric_reader.collection.duration point carries
error.type = java.lang.IllegalStateException. It fails against the current code (the point has
no error.type) and passes with the fix. Full :sdk:metrics:test suite and
:sdk:metrics:spotlessCheck pass; errorprone/NullAway clean.

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Pull request dashboard status

Merged · refreshed 2026-07-28 17:45 UTC

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[jack-berg]

Nice catch. One small comment but looks good otherwise.

[jack-berg]

Suggested change

	// Record the failure on the self-observability sample before it propagates to the
	// outer handler; otherwise error.type is never set and collection failures are
	// reported as successful collections.

The explanation in this comment will make no sense to a future reader since it explains code that is no longer problematic with this change. I would just drop the comment altogether since the code is self explanatory.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.47%. Comparing base (09d6c17) to head (2c53e49).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8650      +/-   ##
============================================
- Coverage     91.64%   91.47%   -0.17%     
- Complexity    10348    10458     +110     
============================================
  Files          1013     1021       +8     
  Lines         27380    27650     +270     
  Branches       3218     3242      +24     
============================================
+ Hits          25092    25293     +201     
- Misses         1558     1615      +57     
- Partials        730      742      +12     

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