GCM: Correctly mark errors as downstream (grafana#97196)

* Correctly mark query type error as downstream

* Correctly mark Prom parsing failures as downstream errors

* Rename function for clarity

Author: aangelisc

pkg/tsdb/cloud-monitoring/cloudmonitoring.go

@@ -443,7 +443,7 @@ func (s *Service) buildQueryExecutors(logger log.Logger, req *backend.QueryDataR
 			}
 			queryInterface = cmp
 		default:
-			return nil, fmt.Errorf("unrecognized query type %q", query.QueryType)
+			return nil, backend.DownstreamError(fmt.Errorf("unrecognized query type %q", query.QueryType))
 		}
 
 		cloudMonitoringQueryExecutors = append(cloudMonitoringQueryExecutors, queryInterface)