AsyncCache support (caffeine 2.7)

[ben-manes]

Micrometer provides convenient bindings for Cache, LoadingCache, and AsyncLoadingCache. AsyncCache was recently split off from AsyncLoadingCache based on integrations where the mapping function isn't provided up front (e.g. Akka's http cache) or for those who need to capture non-key parameters in the given lambda. These behaviors were supported by AsyncLoadingCache but a little clunky.

I believe all that is required is changing CaffeineCacheMetrics from

public static <C extends AsyncLoadingCache> C monitor(...) {}

to

public static <C extends AsyncCache> C monitor(...) {}

Due to the library maintaining strict semver, it should be safe to upgrade the required version clients use.

[izeye]

@ben-manes Thanks for the suggestion! I created #1255 to try to resolve this as suggested.

[shakuzen]

We'll want to note in our 1.2.0 release notes that Caffeine 2.7 or later is now required for the Caffeine caching metrics support.