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
```
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