add /healthz endpoint (#27536)

kuberentes (and Im sure other orchastrators does as well) support two
kind of checks. readiness checks and liveness checks. Grafanas current
`/api/health` endpoint requires database access which might not
always be required for the instance to be considered active.

(cherry picked from commit 6dc73a6712)

pkg/api/api.go

@@ -437,7 +437,8 @@ func (hs *HTTPServer) registerRoutes() {
 	r.Delete("/api/snapshots/:key", reqEditorRole, Wrap(DeleteDashboardSnapshot))
 
 	// Health check
-	r.Get("/api/health", hs.healthHandler)
+	r.Get("/api/health", hs.apiHealthHandler)
+	r.Get("/healthz", hs.healthzHandler)
 
 	r.Get("/*", reqSignedIn, hs.Index)
 }