Refactor Prometheus Metrics

[jonathanio]

Refactor the basic Prometheus metrics handling to:

• Allow a split between web and metrics endpoints, so general requests to the /alive and /healthz metrics in the metrics endpoint don't skew the metrics for the web endpoint.
• Add support for response time summary and quantiles on a per-service basis rather than per-method/path/status.
• Add support for response size metrics.
• Add support for counting in-flight metrics.

Checklist

Before raising or requesting a review of the pull request, please check and confirm the following items have been performed, where possible:

• I have performed a self-review of my code and run any tests locally to check
• I have added tests that prove that any changes are effective and work correctly
• I have made corresponding changes, as needed, to the repository documentation
• Each commit in, and this pull request, have meaningful subjects and bodies for context
• I have added release/..., type/..., and changes/... labels, as needed, to this pull request

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 29 lines in your changes missing coverage. Please review.

Project coverage is 9.74%. Comparing base (06195e9) to head (d7aa62e).
Report is 3 commits behind head on main.

Files	Patch %	Lines
internal/serve/middleware/prometheus.go	0.00%	21 Missing ⚠️
internal/cmd/serve.go	0.00%	3 Missing ⚠️
internal/serve/metrics/main.go	0.00%	3 Missing ⚠️
internal/serve/middleware/metrics.go	0.00%	1 Missing ⚠️
internal/serve/web/main.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             main     #14      +/-   ##
=========================================
- Coverage   10.04%   9.74%   -0.30%     
=========================================
  Files          17      17              
  Lines         498     513      +15     
=========================================
  Hits           50      50              
- Misses        448     463      +15     

Flag	Coverage Δ
unit-tests	9.74% <0.00%> (-0.30%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
internal/serve/middleware/logger.go	0.00% <ø> (ø)
internal/serve/middleware/metrics.go	0.00% <0.00%> (ø)
internal/serve/web/main.go	0.00% <0.00%> (ø)
internal/cmd/serve.go	0.00% <0.00%> (ø)
internal/serve/metrics/main.go	0.00% <0.00%> (ø)
internal/serve/middleware/prometheus.go	0.00% <0.00%> (ø)