remotecfg: Fix errNotModified causing incorrect last_load metric and printing invalid error logs (grafana#2977)

Fixing behavior of errNotModified and remotecfg_last_load_successful metric. This also prevents invalid error log messages from being printed.

Author: zackman0010

internal/service/remotecfg/remotecfg.go

@@ -383,7 +383,7 @@ func (s *Service) fetchRemote() error {
 	if err == nil {
 		s.metrics.lastLoadSuccess.Set(1)
 		s.metrics.lastFetchSuccessTime.SetToCurrentTime()
-	} else {
+	} else if err != errNotModified {
 		s.metrics.totalFailures.Add(1)
 		s.metrics.lastLoadSuccess.Set(0)
 		return err