[Feature Request] Refactor ICache.stats() to return an interface rather than ImmutableCacheStatsHolder

[peteralfonsi]

Is your feature request related to a problem? Please describe

In the backend of the new cache stats API, currently ICache implementations return a concrete class (ImmutableCacheStatsHolder) when providing a fixed snapshot of the cache's stats. We may want to refactor this to use an interface in case we want additional implementations later on.

Describe the solution you'd like

We could create an interface, possibly "CacheStats" or "CacheStatsResponse", which has similar logic of being aggregated by levels at creation time, and also implements Writeable and ToXContent. One downside is that serializing such an interface would be a bit messy, and might require writing class names into the StreamOutput or a similar approach.

Related component

Search:Performance

Describe alternatives you've considered

The status quo also works for now, but we might want to add this flexibility.

Additional context

No response

[peternied]

[Triage - attendees 1 2 3 4 5 6 7 8]
@peteralfonsi Thanks for creating this issue, looking forward to a pull request to address this issue.