fix(metrics): shutdown periodic readers in tests and handle unmapped aggregations (#5157)

[dlowzzxx]

Description

Fixes #5157

Root Cause

1. In opentelemetry-sdk/src/opentelemetry/sdk/metrics/_internal/metric_reader_storage.py, MetricReaderStorage.collect() iterates through view instrument matches and assigns data depending on the aggregation instance. When an unmapped or mock aggregation was encountered, no branch was taken, leaving data unassigned and resulting in UnboundLocalError: cannot access local variable 'data' where it is not associated with a value when constructing Metric(...).
2. In opentelemetry-sdk/tests/metrics/test_periodic_exporting_metric_reader.py, test_exporter_temporality_preference and test_exporter_aggregation_preference initialized a PeriodicExportingMetricReader which started a background daemon thread (_ticker), but never shut down pmr. Similarly, in test_metric_reader_metrics, mp.shutdown() was not guaranteed to execute in a finally block if an assertion failed. These leaking daemon threads continued ticking during subsequent tests (such as test_boundary_statistics and test_creates_view_instrument_matches) and triggered unhandled thread exceptions in the background.

Changes

1. MetricReaderStorage: Added an else: fallback in the aggregation isinstance chain that logs a warning (_logger.warning(...)) and skips the match (continue), preventing unassigned data fallthrough. Removed # pylint: disable=possibly-used-before-assignment.
2. Periodic Exporting Metric Reader Tests: Wrapped reader inspection in try...finally: pmr.shutdown() for test_exporter_temporality_preference and test_exporter_aggregation_preference, and ensured mp.shutdown() (and pmr.shutdown() if not yet stopped) runs inside a finally block in test_metric_reader_metrics.
3. New Test: Added test_collect_skips_unsupported_aggregation to test_metric_reader_storage.py to assert that unsupported aggregations log a warning and are cleanly skipped without raising UnboundLocalError.
4. Changelog Fragment: Added .changelog/5622.fixed (matching towncrier and CI PR number requirements).

Testing

• uv run pytest opentelemetry-sdk/tests/metrics/test_metric_reader_storage.py passed (18 passed).
• uv run pytest opentelemetry-sdk/tests/metrics/test_periodic_exporting_metric_reader.py passed (13 passed, 1 skipped for fork).
• Full metrics test suite uv run pytest opentelemetry-sdk/tests/metrics/ passed (322 passed, 0 failures).
• uv run towncrier check --compare-with origin/main passed cleanly.
• uv run ruff check and uv run ruff format --check passed with zero violations.

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• ✅ login: dlowzzxx / name: dlowzzxx (1362fa4, 1a2c139, 2ec9a74)

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[ocelotl]

please sign the CLA

[ocelotl]

how does this PR fix #5157?

[dlowzzxx]

@ocelotl Thank you for reviewing!

Regarding how this PR addresses #5157:

The traceback captured in #5157 shows two symptoms occurring together:

1. \AssertionError: 2 != 1\ in \TestMetricReaderStorage.test_creates_view_instrument_matches\
2. \PytestUnhandledThreadExceptionWarning: Exception in thread OtelPeriodicExportingMetricReader: UnboundLocalError: cannot access local variable 'data' where it is not associated with a value\ in \metric_reader_storage.py:219.

These two failures share a single root cause:

• Tests in \ est_periodic_exporting_metric_reader.py\ spawned \PeriodicExportingMetricReader\ instances whose daemon background ticker threads (_ticker) were not explicitly joined/shut down in test teardown.
• While \TestMetricReaderStorage\ was executing, a leaked background ticker thread concurrently invoked \collect()\ -> \storage.consume_measurement(). This triggered _ViewInstrumentMatch\ a second time, causing \AssertionError: 2 != 1. Furthermore, because the mock aggregations in that test did not produce a matching aggregation case in \MetricReaderStorage.collect(), the concurrent call referenced an unassigned \data\ variable, raising \UnboundLocalError.

This PR fixes both:

1. Adds explicit
   eader.shutdown()\ in \inally\ blocks across \ est_periodic_exporting_metric_reader.py, preventing the background ticker thread leak.
2. Adds defensive fallback in \MetricReaderStorage.collect()\ so unmapped aggregations do not crash with \UnboundLocalError.

[dlowzzxx]

/dashboard route:reviewers

[opentelemetry-pr-dashboard]

@dlowzzxx, this pull request was routed to reviewers.