refactor: default to prometheus.DefaultRegisterer

[lidel]

This PR contains changes on top of #718

It replaces prometheus.NewRegistry() calls with prometheus.DefaultRegisterer to ensure that by default boxo users who do not specify an explicit registry are not missing some of the metrics.

This convention is what go-libp2p does already, and I feel boxo should follow, to remove confusion why some metrics are present, and some are missing.

TODO

• merge feat(routing/http/server): expose prometheus metrics  #718 and rebase this on main branch
• sharness tests will fail until fix from chore: update to boxo without goprocess kubo#10567 lands in Kubo master
• refactor so boxo modules won't panic if prometheus.DefaultRegisterer is used and tests are run in parallel
  • log error where possible
  • use unique prefix for metric name where not possible to avoid registry.MustRegister

[codecov]

Codecov Report

Attention: Patch coverage is 76.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 60.33%. Comparing base (8673560) to head (6b292d9).

Files with missing lines	Patch %	Lines
gateway/backend_car.go	72.72%	2 Missing and 1 partial ⚠️
gateway/blockstore.go	0.00%	3 Missing ⚠️

@@            Coverage Diff             @@
##             main     #722      +/-   ##
==========================================
+ Coverage   60.31%   60.33%   +0.02%     
==========================================
  Files         244      244              
  Lines       31034    31036       +2     
==========================================
+ Hits        18717    18725       +8     
+ Misses      10644    10639       -5     
+ Partials     1673     1672       -1     

Files with missing lines	Coverage Δ
gateway/metrics.go	82.85% <100.00%> (+0.28%)	⬆️
routing/http/server/server.go	74.57% <100.00%> (+0.26%)	⬆️
gateway/backend_car.go	50.00% <72.72%> (-0.47%)	⬇️
gateway/blockstore.go	0.00% <0.00%> (ø)

... and 14 files with indirect coverage changes

[lidel]

I think the approach of special-casing tests is too tedious, especially if people elaready use boxo modules in their tests, we will be breaking them forcing people to add similar wrappers.

We have a different approach in boxo/gateway/metrics where we catch "duplicate registration" error and log error instead. This way all tests can run in parallel without extra wrapper.

So next steps here:

• undo changes to tests and refactor to use approach from gateway

[lidel]

Pushed refactored version.

Boxo will no longer panic when prometheus.DefaultRegisterer is used, and someone runs ours (or theirs) tests in parallel.

This way

• things work: boxo users are never missing metrics when they run with defaults
• ux remains good: existing boxo users will not have to refactor their existing tests due to vague panic occuring after boxo update