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/post_layer.go

@@ -49,9 +49,9 @@ func (s LocalCachePostStore) ClearCaches() {
 	s.PostStore.ClearCaches()
 
 	if s.rootStore.metrics != nil {
-		s.rootStore.metrics.IncrementMemCacheInvalidationCounter("Last Post Time - Purge")
-		s.rootStore.metrics.IncrementMemCacheInvalidationCounter("Last Posts Cache - Purge")
-		s.rootStore.metrics.IncrementMemCacheInvalidationCounter("Posts Usage Cache - Purge")
+		s.rootStore.metrics.IncrementMemCacheInvalidationCounter(s.rootStore.lastPostTimeCache.Name())
+		s.rootStore.metrics.IncrementMemCacheInvalidationCounter(s.rootStore.postLastPostsCache.Name())
+		s.rootStore.metrics.IncrementMemCacheInvalidationCounter(s.rootStore.postsUsageCache.Name())
 	}
 }
 
@@ -65,8 +65,8 @@ func (s LocalCachePostStore) InvalidateLastPostTimeCache(channelId string) {
 	s.PostStore.InvalidateLastPostTimeCache(channelId)
 
 	if s.rootStore.metrics != nil {
-		s.rootStore.metrics.IncrementMemCacheInvalidationCounter("Last Post Time - Remove by Channel Id")
-		s.rootStore.metrics.IncrementMemCacheInvalidationCounter("Last Posts Cache - Remove by Channel Id")
+		s.rootStore.metrics.IncrementMemCacheInvalidationCounter(s.rootStore.lastPostTimeCache.Name())
+		s.rootStore.metrics.IncrementMemCacheInvalidationCounter(s.rootStore.postLastPostsCache.Name())
 	}
 }