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/api/admin_users.go

@@ -34,7 +34,7 @@ func AdminCreateUser(c *models.ReqContext, form dtos.AdminCreateUserForm) {
 		return
 	}
 
-	metrics.M_Api_Admin_User_Create.Inc()
+	metrics.MApiAdminUserCreate.Inc()
 
 	user := cmd.Result