feat(observability): unify metrics with deco-cx/deco semconv contract

[nicacioliveira]

What

Align @decocms/start (tanstack) telemetry with the deco-cx/deco framework so both frameworks emit the same metric names, units, and attribute keys — required before mixing their data in the shared ClickStack otel_* tables.

Changes

• Durations → seconds at the source (was ms). http.server.request.duration, http.client.request.duration, deco.loader.duration, deco.cms.resolve.duration now divide by 1000 at record time. Removed the "convert in the collector/query layer" stance — we normalize at origin (matches deco).
• HTTP server attributes → semconv: method→http.request.method, route_pattern→http.route, status→http.response.status_code. Proprietary extras namespaced: deco.http.status_class / .outcome / .region, deco.cache.decision / .layer.
• Cache collision fixed: deco.cache.hits + deco.cache.misses → single deco.cache.lookups counter dimensioned by deco.cache.result. This both unifies with deco-cx/deco and removes a deco.cache.hits name-with-different-semantics collision (deco counted all lookups; tanstack counted only hits).
• Loader labels namespaced under deco.* (deco.loader.name, deco.cache.result).

Compatibility

• The caller API is unchanged — RequestMetricLabels keeps its input field names (route_pattern, cache_decision, cache_layer); only the emitted attribute keys changed. No call sites needed edits.
• Clean rename, no dual-emit (ClickStack is a fresh backend).
• npm run typecheck clean; observability.test.ts 14/14 green.

Companion to deco-cx/deco#1211 and deco-cx/apps#1625. Part of the HyperDX → ClickStack migration; conforms to the shared OBSERVABILITY_CONVENTIONS contract.

🤖 Generated with Claude Code

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Summary by cubic

Unifies @decocms/start telemetry with deco-cx/deco semantic conventions so both emit identical metric names, units, and attributes in ClickStack otel_* tables. Durations now record in seconds and HTTP labels use OTel keys; caller API stays the same.

• Refactors

  • Record http.server, http.client, loader, and CMS resolve durations in seconds.
  • Switch to OTel HTTP keys: http.request.method, http.route, http.response.status_code; move extras under deco.* (e.g., deco.http.status_class, deco.cache.decision, deco.http.region).
  • Namespace loader/cache labels: deco.loader.name, deco.cache.result, deco.cache.layer, deco.cache.profile.

• Bug Fixes

  • Replace deco.cache.hits/deco.cache.misses with a single deco.cache.requests counter dimensioned by deco.cache.status to remove a naming collision and match deco-cx/deco.

^{Written for commit be66958. Summary will update on new commits.}