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/dataproxy.go

@@ -8,7 +8,7 @@ import (
 )
 
 func (hs *HTTPServer) ProxyDataSourceRequest(c *m.ReqContext) {
-	c.TimeRequest(metrics.M_DataSource_ProxyReq_Timer)
+	c.TimeRequest(metrics.MDataSourceProxyReqTimer)
 
 	dsId := c.ParamsInt64(":id")
 	ds, err := hs.DatasourceCache.GetDatasource(dsId, c.SignedInUser, c.SkipCache)