Collect and provide cache hit statistics

[anders-b-jansson]

Description As docuum anyway listens to events, could it also count how many times each image & layer has hit/missed?

Alternatives considered N/A

Additional context For people respsonsible for dimensioning storage, cache sizes, build & runtime systems, this kind of stats could be valueable for tuning.

[stepchowfun]

That seems like it would be useful. I would review a PR for it if anyone has time.