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

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.

Project coverage is 60.37%. Comparing base (1bf1488) to head (edd18aa).

Files with missing lines	Patch %	Lines
gateway/blockstore.go	0.00%	1 Missing ⚠️
routing/http/server/server.go	0.00%	1 Missing ⚠️

@@            Coverage Diff             @@
##             main     #722      +/-   ##
==========================================
+ Coverage   60.34%   60.37%   +0.03%     
==========================================
  Files         243      243              
  Lines       31039    31044       +5     
==========================================
+ Hits        18730    18744      +14     
+ Misses      10644    10633      -11     
- Partials     1665     1667       +2     

Files with missing lines	Coverage Δ
examples/car-file-fetcher/main.go	42.85% <100.00%> (+3.96%)	⬆️
gateway/backend_car.go	50.92% <100.00%> (+0.46%)	⬆️
gateway/blockstore.go	0.00% <0.00%> (ø)
routing/http/server/server.go	74.44% <0.00%> (+0.22%)	⬆️

... and 9 files with indirect coverage changes

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