MM-57193: Standardize the cache names (#26441)

We use the same name for hit/miss/invalidation metrics.

The downside is that now we have conflated a full cache purge
and a single cache key invalidation into a single label, but
the advantage is that it's easier to monitor the usage of a single
cache.

And moreover, duplicating the same string across multiple places
increases the chances of making a mistake. Centralizing the cache
name only at instantiation simplifies the code.

After this, we would be ready to use Redis.

https://mattermost.atlassian.net/browse/MM-57193

```release-note
NONE
```

server/channels/store/localcachelayer/file_info_layer.go

@@ -53,7 +53,7 @@ func (s LocalCacheFileInfoStore) GetForPost(postId string, readFromMaster, inclu
 func (s LocalCacheFileInfoStore) ClearCaches() {
 	s.rootStore.fileInfoCache.Purge()
 	if s.rootStore.metrics != nil {
-		s.rootStore.metrics.IncrementMemCacheInvalidationCounter("File Info Cache - Purge")
+		s.rootStore.metrics.IncrementMemCacheInvalidationCounter(s.rootStore.fileInfoCache.Name())
 	}
 }
 
@@ -64,7 +64,7 @@ func (s LocalCacheFileInfoStore) InvalidateFileInfosForPostCache(postId string,
 	}
 	s.rootStore.doInvalidateCacheCluster(s.rootStore.fileInfoCache, cacheKey, nil)
 	if s.rootStore.metrics != nil {
-		s.rootStore.metrics.IncrementMemCacheInvalidationCounter("File Info Cache - Remove by PostId")
+		s.rootStore.metrics.IncrementMemCacheInvalidationCounter(s.rootStore.fileInfoCache.Name())
 	}
 }