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/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
 }