Cleanup storage cache metrics

[SuperQ]

Split request and hit metrics into separate metrics, rather than using
labels. This avoids duplication of data and makes metric math easier.

• Count cache errors separately to avoid weird math.
• Hit ratio: registry_storage_cache_hits_total / registry_storage_cache_requests_total
• Miss ratio: 1 - (registry_storage_cache_hits_total / registry_storage_cache_requests_total
• Misses: registry_storage_cache_requests_total - registry_storage_cache_hits_total

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "cache_metrics" git@github.com:SuperQ/distribution.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[codecov]

Codecov Report

Merging #2897 into master will decrease coverage by 0.22%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2897      +/-   ##
==========================================
- Coverage   60.69%   60.46%   -0.23%     
==========================================
  Files         102      102              
  Lines        8044     8000      -44     
==========================================
- Hits         4882     4837      -45     
- Misses       2512     2516       +4     
+ Partials      650      647       -3

Flag	Coverage Δ
#linux	60.46% <ø> (-0.23%)	⬇️

Impacted Files	Coverage Δ
registry/storage/blobcachemetrics.go	53.84% <ø> (+1.99%)	⬆️
registry/client/blob_writer.go	45.23% <0%> (-8.26%)	⬇️
registry/client/repository.go	57.82% <0%> (-1.94%)	⬇️
registry/handlers/blobupload.go	44.72% <0%> (-1.09%)	⬇️
notifications/metrics.go	96.66% <0%> (-0.48%)	⬇️
registry/registry.go	32% <0%> (-0.29%)	⬇️
registry/client/auth/session.go	68.69% <0%> (+0.63%)	⬆️
registry/handlers/manifests.go	53.64% <0%> (+0.81%)	⬆️

---

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[SuperQ]

@manishtomar, can you also look at this one?

[milosgajdos]

@SuperQ would you mind rebasing? Feel free to close if you no longer want to work on this.

[SuperQ]

@milosgajdos rebase done. Updated to also split out the errors into a separate metric. Things get weird when you have to figure out hits/misses/errors that way.

I wonder if it would be a good idea to migrate away from the docker/go-metrics wrapper package. It seems abandoned.

[milosgajdos]

I wonder if it would be a good idea to migrate away from the docker/go-metrics wrapper package. It seems abandoned.

@SuperQ that's a good question. Normally I'd say why not, but do we have any extra requirements that are not met by that module? I dont think we do

[SuperQ]

I don't think the wrapper library adds much value.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01% ⚠️

Comparison is base (807a836) 55.95% compared to head (c19adfd) 55.94%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2897      +/-   ##
==========================================
- Coverage   55.95%   55.94%   -0.01%     
==========================================
  Files         110      110              
  Lines       11050    11048       -2     
==========================================
- Hits         6183     6181       -2     
  Misses       4178     4178              
  Partials      689      689              

Files Changed	Coverage Δ
...egistry/storage/cache/cachedblobdescriptorstore.go	58.13% <100.00%> (-1.87%)	⬇️

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[deleteriousEffect]

These look like solid changes to me 🚀