oss_11_m2_per_reader_cardinality_limit

[ocelotl]

Closes #41

Per-MetricReader default cardinality limit (Linear OSS-11, finding M2), layered on the M1 base limit. Optional cardinality_limit on MetricReader / PeriodicExportingMetricReader / InMemoryMetricReader, threaded through the pipeline into _ViewInstrumentMatch; unset falls back to the M1 base default (2000); non-positive raises ValueError.

Stacked on oss_11_m1_metric_cardinality_limit (PR #17) — this PR's base is that branch, so its diff shows only the M2 delta. Retarget to the fork default branch once M1 merges.

Validation: M2 delta scope contained to opentelemetry-sdk + changelog; full metrics suite 310 passed.

Linear issue: https://linear.app/dash0/issue/OSS-11/m1-m3-metric-cardinality-limits-base-per-reader-per-view

[ocelotl]

🔒 Internal (dash0) — not for upstream.

• Linear: https://linear.app/dash0/issue/OSS-11/m1-m3-metric-cardinality-limits-base-per-reader-per-view (finding M2)
• Closes: oss_11_m2_per_reader_cardinality_limit #41 · Branch: oss_11_m2_per_reader_cardinality_limit · Commit: b2a00b40d · STACKED on oss_11_m1_metric_cardinality_limit.
• Validation: M2 delta scope sdk + changelog; full metrics suite 310 passed.

[ocelotl]

📣 Public-facing draft — to be used for the upstream PR in open-telemetry/opentelemetry-python. No internal references; copy verbatim.

Title: Add per-MetricReader default cardinality limit

What

Adds an optional cardinality_limit parameter to MetricReader and its concrete subclasses (PeriodicExportingMetricReader, InMemoryMetricReader). When set, the limit is threaded through the SDK pipeline (SynchronousMeasurementConsumer → MetricReaderStorage → _ViewInstrumentMatch) and applied to every metric stream produced through that reader's pipeline. When unset, the existing base default cardinality limit is used. A non-positive limit raises ValueError.

Why

The base cardinality limit is fixed and cannot be tuned per reader; different export pipelines often need different cardinality/memory trade-offs. This aligns with the spec's configurable cardinality limits and the reader-level defaults in Go (sdk/metric/pipeline.go) and Java (CardinalityLimitSelector).

Spec / references

specification/metrics/sdk.md — Cardinality limits; Go sdk/metric/pipeline.go; Java CardinalityLimitSelector.

Notes

Builds on the base cardinality-limit change (_DEFAULT_CARDINALITY_LIMIT + the otel.metric.overflow series in _ViewInstrumentMatch). This adds only the per-reader override; when a reader sets no limit, behavior is unchanged.

Tests

New integration tests: a reader with a small limit overflows at that limit; no overflow below the reader's limit; unset falls back to the base default; non-positive limits are rejected. Existing metrics tests pass (310).