Cache metrics for http clients

Both httpclient from Apache and OkHttp (which are instrumented by micrometer: OkHttpMetricsEventListener and https://github.com/micrometer-metrics/micrometer/issues/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: https://github.com/micrometer-metrics/micrometer/compare/master...kubamarchwicki:httpclient-cache-metrics. If that's something interesting, I'd hapilly finalize both OkHttp as well as httpclient.