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/org_invite.go

@@ -188,8 +188,8 @@ func (hs *HTTPServer) CompleteInvite(c *m.ReqContext, completeInvite dtos.Comple
 
 	hs.loginUserWithUser(user, c)
 
-	metrics.M_Api_User_SignUpCompleted.Inc()
-	metrics.M_Api_User_SignUpInvite.Inc()
+	metrics.MApiUserSignUpCompleted.Inc()
+	metrics.MApiUserSignUpInvite.Inc()
 
 	return Success("User created and logged in")
 }