oss_11_m3_per_view_cardinality_limit

[ocelotl]

Closes #48

Per-View cardinality-limit override (Linear OSS-11, finding M3), the finest layer. Optional View(aggregation_cardinality_limit=...); precedence View > per-reader default (M2) > base default 2000 (M1), resolved centrally in _ViewInstrumentMatch.

Stacked on oss_11_m2_per_reader_cardinality_limit (PR #44) → its base. Retarget once M1/M2 merge. Full M1→M2→M3 chain: PR #17 ← #44 ← this.

Validation: M3 delta scope contained to opentelemetry-sdk + changelog; full metrics suite 315 passed (M1/M2 tests intact).

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 M3)
• Closes: oss_11_m3_per_view_cardinality_limit #48 · Branch: oss_11_m3_per_view_cardinality_limit · Commit: b1eed5255 · STACKED on oss_11_m2_....
• Validation: M3 delta scope sdk + changelog; full metrics suite 315 passed. Completes OSS-11 (M1/M2/M3).

[ocelotl]

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

Title: Add per-View cardinality-limit override

What

Adds an optional aggregation_cardinality_limit parameter to the View class. When set, it caps the number of distinct attribute sets (metric points) tracked for the streams matched by that view, folding excess into the standard {"otel.metric.overflow": True} series.

Why

Completes the cardinality-limit configuration surface described by the metrics spec. Previously the limit could be set only globally (base default) or per-MetricReader; there was no way to tune an individual metric stream. This lets a single high-cardinality instrument be capped independently of the reader-wide default.

Precedence

View override > per-reader default > base default (2000). An unset View override falls through to the reader default and then the base default, so existing behavior is unchanged when the new parameter is not used. Precedence is resolved in one place (_ViewInstrumentMatch).

Spec

specification/metrics/sdk.md — Cardinality limits (per-instrument/View configuration). Mirrors the Java SDK's per-instrument cardinality-limit selection.

Builds on

The base default and per-reader default cardinality-limit changes.

Tests

New unit tests: a View with a small limit overflows at that limit; overrides a larger reader default; overrides an unset reader; unset View falls through to reader then base default. Full metrics suite passes (315), including base and per-reader cardinality tests.