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/search.go

@@ -61,6 +61,6 @@ func Search(c *m.ReqContext) Response {
 		return Error(500, "Search failed", err)
 	}
 
-	c.TimeRequest(metrics.M_Api_Dashboard_Search)
+	c.TimeRequest(metrics.MApiDashboardSearch)
 	return JSON(200, searchQuery.Result)
 }