Don't promote service.version to a per-series metric label (OTLP→Prometheus: version belongs on target_info/build_info)

Description

Summary

service.version is set on the OTLP metrics resource (the MeterProvider’s Resource), so it is promoted to a service_version label on every emitted metric series. Per the OpenTelemetry → Prometheus compatibility spec this is a deviation: version belongs on an info metric (target_info from the resource, or a *_build_info gauge), never as a label on ordinary series.

The standard

Only service.name (+ service.namespace) → job and service.instance.id → instance are meant to become labels. Every other resource attribute — including service.version — goes to a target_info metric (OpenMetrics 1.0 convention). Promoting a resource attribute to per-series labels is a documented, non-default opt-in.

Consequence of putting version on every series: each new build mints a whole new series set. After a redeploy, the old-version and new-version series coexist for the query-lookback window, so any sum-style panel adds both → a transient multiplier; and active-series cardinality grows with the number of versions ever seen. This was found loudly on graph2otel (which runs a per-commit :main image) — see rknightion/graph2otel#104 for the full spec analysis. Repos on stable release tags hit the cardinality growth but rarely see the doubling.

Change

• Keep service.version on the OTel resource (semconv-correct → flows to target_info).
• Stop it becoming a per-series label: rely on the existing *_build_info{version=...} gauge (and/or target_info) for version, joined via group_left where a panel needs it.
• Audit dashboards/alerts for any query that aggregates over or filters on service_version and repoint them at the info metric.

Imported from GitHub issue #82 on 2026-08-14, when this repo migrated from GitHub Issues to Backlog.md. The original issue has been deleted; its verbatim body, labels and comments are preserved in archive/issues-dump.json (jq '.[] | select(.number == 82)' archive/issues-dump.json).

Cross-repo consistency pass with rknightion/graph2otel#104 (detailed spec citations there). Sibling issues: tailscale2otel#187, opnsense-exporter#270.

Acceptance Criteria

• #1 service_version no longer appears as a label on ordinary metric series (only on target_info / *_build_info)
• #2 Version stays queryable via the build-info gauge (+ a documented group_left example)
• #3 Dashboards/alerts updated if any referenced service_version
• #4 Note the OTLP→Prometheus resource-attribute convention in the telemetry setup docs

Definition of Done

• #1 just gate is green (ruff check + ruff format –check + mypy src + pytest) — run it, don’t assert it
• #2 just gen-config run and its output committed, if config.py changed (CI drift gate fails otherwise)
• #3 committed straight to main with a conventional-commit message, and pushed