fix(spanner): use current export time for gauge intervals instead of static meter start time

Author: sakthivelmanii

spanner/metrics_monitoring_exporter.go

@@ -359,7 +359,7 @@ func toNonemptyTimeIntervalpb(start, end time.Time, isGauge bool) (*monitoringpb
 	// previous interval, for all non-gauge types.
 	// https://cloud.google.com/monitoring/api/ref_v3/rpc/google.monitoring.v3#timeinterval
 	if isGauge {
-		end = start
+		start = end
 	} else if end.Sub(start).Milliseconds() <= 1 {
 		end = start.Add(time.Millisecond)
 	}