Metrics: use consistent naming for exported variables (#18134)

* Metrics: remove unused metrics

Metric `M_Grafana_Version` is not used anywhere, nor the mentioned
`M_Grafana_Build_Version`. Seems to be an artefact?

* Metrics: make the naming consistent

* Metrics: add comments to exported vars

* Metrics: use proper naming

Fixes #18110

pkg/services/sqlstore/dashboard.go

@@ -87,7 +87,7 @@ func saveDashboard(sess *DBSession, cmd *m.SaveDashboardCommand) error {
 		dash.CreatedBy = userId
 		dash.Updated = time.Now()
 		dash.UpdatedBy = userId
-		metrics.M_Api_Dashboard_Insert.Inc()
+		metrics.MApiDashboardInsert.Inc()
 		affectedRows, err = sess.Insert(dash)
 	} else {
 		dash.SetVersion(dash.Version + 1)