MM-27199 - Add metrics to MM server for total enabled user count (#15116)

* Add metrics to MM server for total enabled user count and include installation ID from a new env var

* diagnostics context

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

imports/placeholder.go

@@ -15,4 +15,7 @@ import (
 
 	// This is a placeholder so this package can be imported in Team Edition when it will be otherwise empty.
 	_ "github.com/mattermost/mattermost-server/v5/jobs/expirynotify"
+
+	// This is a placeholder so this package can be imported in Team Edition when it will be otherwise empty.
+	_ "github.com/mattermost/mattermost-server/v5/jobs/active_users"
 )