feat: add OpenTelemetry metrics middleware alongside go-metrics

[adityakrmishra]

Addresses #439

Hi team,

Following up on the discussion in #439, this PR introduces OpenTelemetry as an alternative metrics layer. It runs entirely parallel to the existing rcrowley/go-metrics implementation to ensure zero disruption to internal Palantir systems that rely on the legacy metrics.

What this solves:
The legacy go-metrics lacks label/tag support, forcing high-cardinality data like installation_id into the metric names. This new OTel middleware maps the exact same data points but uses proper OTel attributes (e.g., github.rate.remaining{installation.id="42"}).

Changes:

• Added go.opentelemetry.io/otel/metric to go.mod (API only, keeping it lightweight without SDK bloat).
• Added githubapp/middleware_otel.go containing the parallel implementation.
• Zero modifications to existing files (middleware.go, dispatcher.go, scheduler.go).

New Entry Points:
Users can now opt-in via:

• OTelClientMetrics(mp)
• OTelErrorCallback(mp)
• OTelAsyncErrorCallback(mp)
• WrapSchedulerWithOTel(inner, mp)

All existing tests pass, and the code has been written to match the idiomatic style and comment density of the current project. Let me know if any adjustments are needed!

[palantirtech]

Thanks for your interest in palantir/go-githubapp, @adityakrmishra! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

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Description

feat: add OpenTelemetry metrics middleware alongside go-metrics

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