oss_11_m3_per_view_cardinality_limit

[ocelotl]

Metric cardinality limits M3: per-View cardinality-limit override, layered on the M1 base + M2 per-reader default (Linear OSS-11, finding M3).

Branch / worktree: oss_11_m3_per_view_cardinality_limit
Stacked on: oss_11_m2_per_reader_cardinality_limit (PR #44).

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)
• Slug/branch/worktree: oss_11_m3_per_view_cardinality_limit
• Commit: b1eed5255 · PR: oss_11_m3_per_view_cardinality_limit #49 (STACKED, base = oss_11_m2_per_reader_cardinality_limit)
• Validation: M3 delta scope sdk + changelog; full metrics suite 315 passed.
• Reviewer note: Completes OSS-11 (M1→M2→M3). Full stack: PR oss_11_m1_metric_cardinality_limit #17 (M1) ← PR oss_11_m2_per_reader_cardinality_limit #44 (M2) ← this (M3). Retarget each to the fork default branch as the one below merges. With M3 done, OSS-11 is fully implemented — recommend Diego mark OSS-11 Done once the stack merges.

[ocelotl]

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

Title: Metrics SDK: support per-View cardinality-limit override (View.aggregation_cardinality_limit)

The metrics spec lets the cardinality limit be configured at multiple granularities. The SDK supports a base default (2000) and a per-MetricReader default, but there is no way to set a cardinality limit for an individual metric stream via a View. The spec's cardinality-limits section allows per-instrument configuration via the View mechanism (the finest-grained control). Without it, a user cannot cap a single high-cardinality instrument tighter/looser than the reader-wide default.

Spec: specification/metrics/sdk.md — Cardinality limits. Reference: the Java SDK exposes per-instrument limits (View-level / CardinalityLimitSelector) with View > reader > default precedence.

Proposed fix

Add an optional aggregation_cardinality_limit argument to View. When set, it overrides both the per-reader default and the base default for the streams matched by that view; when unset, resolution falls through to the reader default and then the base default (2000). The value is threaded into the _ViewInstrumentMatch the view produces. (Assumes the base default and per-reader default limits already exist.)