Cache metrics for http clients

[kubamarchwicki]

Both httpclient from Apache and OkHttp (which are instrumented by micrometer: OkHttpMetricsEventListener and #533) support provider driven caching (based on http headers)

It would be helpful (for the sake of completeness) for micrometer to provide insight to the state of caches (size, maxSize, hit and misses).

I've drafted a cache metrics for OkHttp which I'd like to get feedback ok: master...kubamarchwicki:httpclient-cache-metrics.
If that's something interesting, I'd hapilly finalize both OkHttp as well as httpclient.

[shakuzen]

@kubamarchwicki thank you for opening this issue. It sounds like a good addition to metrics provided. I took a quick look at the draft implementation. It looks like it is on the right track. Please open a pull request and we can proceed with getting this integrated.

[marcingrzejszczak]

hey @kubamarchwicki, long time no see :) What about the PR, are you willing to file it?