This originated from https://github.com/mattermost/mattermost/issues/15249.
However, the original idea was discarded https://github.com/mattermost/mattermost/issues/15249#issuecomment-709713065
as being too complicated to implement. Then I had another
idea to implement it just for session objects.
My thinking was that since every single request allocates a new
session struct, it would be good to use a sync.Pool for that.
However, 4 years later, now we know that the primary bottleneck
in app performance comes from websocket event marshalling.
Therefore, while it would be good to do this, it is difficult
to do it correctly (as shown by the numerous racy tests).
Hence, reverting this.
```release-note
NONE
```
* Add platform information for push notification metrics
* Address feedback
* Add the client platform returned by the devices to the normalize function
* Add "no platform" platform label to distinguish from unknown
* [MM-57066] Add metric counters for notification events
* Some small changes
* Account for Metrics() sometimes being nil
* Fix test (again)
* Fix more tests
* A few changes from testing - added success counter
* Missed a mock
* Lint
* Add feature flag for notification monitoring
It was a good decision in hindsight to keep the public module as 0.x
because this would have been a breaking change again.
https://mattermost.atlassian.net/browse/MM-53032
```release-note
Changed the Go module path from github.com/mattermost/mattermost-server/server/v8 to github.com/mattermost/mattermost/server/v8.
For the public facing module, it's path is also changed from github.com/mattermost/mattermost-server/server/public to github.com/mattermost/mattermost/server/public
```
