Add cortex_frontend_query_result_cache_requests_total and cortex_frontend_query_result_cache_hits_total metrics

[pracucci]

What this PR does

In #5212 I've added the optional and experimental support to lookup cardinality queries from the cache. However, we don't have a way to track the cache hit ratio specifically for the cardinality API endpoints. Reason is that today we measure the query-frontend cache hit ratio looking at the underlying memcache metrics (e.g. thanos_cache_memcached_hits_total / thanos_cache_memcached_requests_total) which are not split by request type / route (and not easy to do).

In this PR I propose to add a couple of new metrics which specifically track the query results cache cache hit ratio. I haven't changed the "Mimir / Queries" dashboard yet because I would like to see the new metrics deployed first before changing the dashboard (changes will be backward compatible).

Note to reviewers:

• The new metrics have been prefixed by cortex_frontend_query_result_cache_  to keep it consistent with other metrics we already have
• At this level we don't have route and propagating it would be quite tedious. On the other hand, I would prefer to not add logic to detect the route from the request URL, so I went to a simpler path which is a simple hardcoded request_type whose values could be query_range or cardinality.

Which issue(s) this PR fixes or relates to

N/A

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[pstibrany]

Looks fine.