oss_11_m1_metric_cardinality_limit

[ocelotl]

Closes #16

Implements M1 (base layer) of metric cardinality limits (Linear OSS-11, findings M1–M3): _ViewInstrumentMatch now enforces a default aggregation cardinality limit (2000). Once the number of tracked attribute sets would exceed the limit, previously-unseen attribute sets are aggregated into a single overflow series {'otel.metric.overflow': True} (one slot reserved so total points never exceed the limit), matching the Go/Java reference implementations. A warning is logged once when overflow first occurs.

M2 (per-MetricReader default) and M3 (per-View override) remain and will be tracked as their own stacked PRs.

Validation: contained to the metrics SDK + tests + changelog; new cardinality test (3 passed) and full metrics suite (306 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 (M1 of M1–M3)
• Closes: oss_11_m1_metric_cardinality_limit #16 · Branch/worktree: oss_11_m1_metric_cardinality_limit · Commit: 256db0546
• Validation: cardinality test 3 passed; full metrics suite 306 passed; scope contained to metrics SDK + tests + changelog.
• Remaining: M2 (per-reader) and M3 (per-view) to follow as stacked PRs.

[ocelotl]

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

Title: Enforce a default metric aggregation cardinality limit with an overflow series

What

_ViewInstrumentMatch now enforces a default aggregation cardinality limit of 2000. Once the number of tracked attribute sets would exceed the limit, previously-unseen attribute sets are aggregated into a single overflow series identified by {"otel.metric.overflow": true} (one slot is reserved so the total number of points never exceeds the limit). A warning is logged once, the first time overflow occurs.

Why

The metrics SDK previously tracked every distinct attribute set without bound and emitted no overflow series, so an attribute-value explosion grew memory without limit. This implements the base ("M1") layer of the spec's cardinality-limit requirement, matching the Go and Java reference implementations.

Scope

This is the base default limit only. Follow-ups will add a per-MetricReader default and a per-View override.

Testing

New integration test for the cardinality limit + overflow behavior; full metrics suite passes (306).