feat: add OpenTelemetry metrics (requests, duration, errors)

[maximn]

Summary

Adds optional OpenTelemetry metrics — three instruments (request counter, latency histogram, error counter) published from a Meter named GoogleMapsApi, recorded once per call and zero-cost until a listener subscribes. This is the metrics half of #296; the ILogger half is intentionally left out and documented as a non-goal.

Related issue

Closes #296

Changes

• New GoogleMapsApi.Diagnostics.GoogleMapsMetrics (public MeterName const; internal Meter + gmaps.client.requests / gmaps.client.request.duration / gmaps.client.request.errors instruments) with a zero-cost Record() helper; no new package dependency.
• MapsAPIGenericEngine records metrics once per call in the existing finally, surfacing the HTTP status code via a callback — no behavioural change to request flow, exceptions, or tracing.
• Updated PublicAPI.Unshipped.txt, added hermetic DiagnosticsMetricsTests (MeterListener-based), and documented metrics in README.md + .agents/observability.md (removed the resolved B5 item; recorded the ILogger non-goal).

Test plan

• dotnet build clean on all three TFMs (PublicAPI lock satisfied); dotnet format clean on the new files.
• 256 hermetic unit tests pass on net8.0 and net10.0 — including 3 new metrics tests + the existing tracing tests. Live-network IntegrationTests require a GOOGLE_API_KEY and were not run in this environment.

Checklist

• dotnet format has been run.
• dotnet test passes locally (with a valid GOOGLE_API_KEY for integration tests).
• Multi-framework build passes (netstandard2.0, net8.0, net10.0).
• XML documentation updated if public API surface changed.

[github-actions]

🔬 TestGlance

✅ 265 passed across 1 job — 100.0% · ⏱️ 2.1s

Job	Result	Pass rate	Duration	Health
build	✅	265/265 · 100.0%	2.1s	—	Report

🟡 build — details

✅ 265 passed
████████████████ 100.0%
⏱️ 2.1s

vs master

Metric	master	PR	Delta
Pass rate	100.0%	100.0%	+0.0%
Duration	2.1s	2.1s	-1.1%

📄 HTML Report

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Updated 2026-06-30T21:46:53.390Z