Support Warning header aggregation and reporting in crane

[imjasonh]

opencontainers/distribution-spec#393 seems to be on a good path, so I figured I'd put in a better implementation of warning collection/reporting in ggcr.

• adds an internal transport that collects and reports warnings from registry requests
• adds registry.WithWarning to pkg/registry that probabilistically reports the given warning (WithWarning(0.1, "hi") warns "hi" 10% of the time)

This also updates cmd/registry to add demo warnings some % of the time; I'll probably revert this before merging, this is just for demo purposes. Speaking of demo:

$ go run ./cmd/registry &
$ go run ./cmd/crane cp alpine localhost:1338/alpine
...
[WARNING]: 60% of the time, it works every time.
[WARNING]: This registry is cool.

Logic for parsing the warning message out of the header is pretty janky, and could use some tests.

Clients that use go-containerregistry won't suddenly start reporting warnings without changes. They'll need to inject a transport like crane does -- if we think crane's approach is generally useful we can make it available in pkg/crane itself.

[codecov-commenter]

Codecov Report

Merging #1604 (2900894) into main (0f2db49) will decrease coverage by 0.07%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #1604      +/-   ##
==========================================
- Coverage   72.80%   72.74%   -0.07%     
==========================================
  Files         118      118              
  Lines        9440     9452      +12     
==========================================
+ Hits         6873     6876       +3     
- Misses       1869     1878       +9     
  Partials      698      698              

Impacted Files	Coverage Δ
pkg/registry/registry.go	83.33% <0.00%> (-16.67%)	⬇️
pkg/v1/remote/options.go	77.77% <ø> (ø)

... and 1 file with indirect coverage changes

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[jonjohnsonjr]

I think there are a couple ways we could go with this that will be slightly better than adding WithWarnCollector to crane and remote.

We already have logs.Warn (used here in this PR currently). We could just have warnings be written to logs.Warn where we set logs.Warn to do warncollectory things.

There's also remote.WithProgress that we currently use to emit very specific kinds of updates. We could do something similar (or even use the same option) to send clients more kinds of events, e.g. these Warnings.

I really dislike the logs package -- it would probably be better if we were just emitting events and having consumers do things with them...

There's also the slog stuff that has its own Warn level, which maybe we could replace the logs package with 🤔

How do you feel about this not being a public API, and instead we have crane pass in a custom transport that does all these warncollectory things until we have a happier public interface?

[imjasonh]

How do you feel about this not being a public API, and instead we have crane pass in a custom transport that does all these warncollectory things until we have a happier public interface?

I'm fine with that. I'll get something along these lines up shortly.