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/datasource.go

@@ -25,7 +25,7 @@ func init() {
 }
 
 func GetDataSourceById(query *m.GetDataSourceByIdQuery) error {
-	metrics.M_DB_DataSource_QueryById.Inc()
+	metrics.MDBDataSourceQueryByID.Inc()
 
 	datasource := m.DataSource{OrgId: query.OrgId, Id: query.Id}
 	has, err := x.Get(&datasource)