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))
 }