* 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
```
