searcher: prometheus gauge for SEARCHER_CACHE_SIZE_MB
Created by: keegancsmith
This is to allow us to create a dashboard which includes the configured maximum size of the cache or make a percentage of cache used. This will make it easier to check admin configurations, since we don't need to check the configuration we can just check the dashboard.
Test Plan: started locally and checked the value of searcher_store_max_cache_size_bytes in the metrics page for searcher.
Depends on https://github.com/sourcegraph/sourcegraph/pull/36161
Part of https://github.com/sourcegraph/sourcegraph/issues/34828