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/alerting/eval_handler.go

@@ -70,5 +70,5 @@ func (e *DefaultEvalHandler) Eval(context *EvalContext) {
 	context.EndTime = time.Now()
 
 	elapsedTime := context.EndTime.Sub(context.StartTime).Nanoseconds() / int64(time.Millisecond)
-	metrics.M_Alerting_Execution_Time.Observe(float64(elapsedTime))
+	metrics.MAlertingExecutionTime.Observe(float64(elapsedTime))
 }

pkg/services/alerting/notifier.go

@@ -57,7 +57,7 @@ func (n *notificationService) sendAndMarkAsComplete(evalContext *EvalContext, no
 	notifier := notifierState.notifier
 
 	n.log.Debug("Sending notification", "type", notifier.GetType(), "uid", notifier.GetNotifierUID(), "isDefault", notifier.GetIsDefault())
-	metrics.M_Alerting_Notification_Sent.WithLabelValues(notifier.GetType()).Inc()
+	metrics.MAlertingNotificationSent.WithLabelValues(notifier.GetType()).Inc()
 
 	err := notifier.Notify(evalContext)
 

pkg/services/alerting/reader.go

@@ -43,6 +43,6 @@ func (arr *defaultRuleReader) fetch() []*Rule {
 		}
 	}
 
-	metrics.M_Alerting_Active_Alerts.Set(float64(len(res)))
+	metrics.MAlertingActiveAlerts.Set(float64(len(res)))
 	return res
 }

pkg/services/alerting/result_handler.go

@@ -44,7 +44,7 @@ func (handler *defaultResultHandler) handle(evalContext *EvalContext) error {
 		annotationData.Set("noData", true)
 	}
 
-	metrics.M_Alerting_Result_State.WithLabelValues(string(evalContext.Rule.State)).Inc()
+	metrics.MAlertingResultState.WithLabelValues(string(evalContext.Rule.State)).Inc()
 	if evalContext.shouldUpdateAlertState() {
 		handler.log.Info("New state change", "alertId", evalContext.Rule.ID, "newState", evalContext.Rule.State, "prev state", evalContext.PrevAlertState)