Add observer metrics

[Kubuxu]

No description provided.

[Kubuxu]

@rvagg I would appriciate a post-review, I'm going to merge this when CI passes.

[Copilot]

Pull Request Overview

This pull request adds Prometheus metrics collection to the observer component, enabling monitoring of message processing and rotation operations. The changes introduce OpenTelemetry-based metrics with an optional HTTP endpoint for metrics export.

• Adds OpenTelemetry and Prometheus dependencies for metrics collection
• Implements metrics tracking for message reception and rotation operations
• Creates an optional /debug/metrics HTTP endpoint for Prometheus scraping

Reviewed Changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
observer/metrics.go	New file defining OpenTelemetry metrics for rotations and messages received
observer/observer.go	Integrates metrics collection into message processing and adds optional metrics HTTP endpoint
observer/options.go	Adds configuration option for enabling metrics export
observer/query.go	Removes serveMux method (functionality moved inline)
cmd/f3/observer.go	Enables metrics export by default in observer command
go.mod	Updates OpenTelemetry dependencies and adds Prometheus client

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 23 lines in your changes missing coverage. Please review.

Project coverage is 64.68%. Comparing base (993d8f7) to head (eece8dd).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
observer/observer.go	0.00%	18 Missing ⚠️
observer/options.go	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1042      +/-   ##
==========================================
- Coverage   64.91%   64.68%   -0.24%     
==========================================
  Files          80       80              
  Lines        9733     9751      +18     
==========================================
- Hits         6318     6307      -11     
- Misses       2914     2938      +24     
- Partials      501      506       +5     

Files with missing lines	Coverage Δ
observer/query.go	0.00% <ø> (ø)
observer/options.go	5.69% <0.00%> (-0.12%)	⬇️
observer/observer.go	0.00% <0.00%> (ø)

... and 6 files with indirect coverage changes

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