improve logging on forward failures

flusher.go

@@ -554,15 +554,26 @@ func (s *Server) forward(
 		if ctx.Err() != nil {
 			// We exceeded the deadline of the flush context.
 			span.Add(ssf.Count("forward.error_total", 1, map[string]string{"cause": "deadline_exceeded"}))
+			entry.WithFields(logrus.Fields{
+				"dropped": len(metrics),
+				"cause":   "deadline_exceeded",
+			}).WithError(err).Error("Failed to forward to an upstream Veneur")
 		} else if statErr, ok := status.FromError(err); ok &&
 			(statErr.Message() == "all SubConns are in TransientFailure" || statErr.Message() == "transport is closing") {
 			// We could check statErr.Code() == codes.Unavailable, but we don't know all of the cases that
 			// could return that code. These two particular cases are fairly safe and usually associated
 			// with connection rebalancing or host replacement, so we don't want them going to sentry.
 			span.Add(ssf.Count("forward.error_total", 1, map[string]string{"cause": "transient_unavailable"}))
+			entry.WithFields(logrus.Fields{
+				"dropped": len(metrics),
+				"cause":   "transient_unavailable",
+			}).WithError(err).Error("Failed to forward to an upstream Veneur")
 		} else {
 			span.Add(ssf.Count("forward.error_total", 1, map[string]string{"cause": "send"}))
-			entry.WithError(err).Error("Failed to forward to an upstream Veneur")
+			entry.WithFields(logrus.Fields{
+				"dropped": len(metrics),
+				"cause":   "send",
+			}).WithError(err).Error("Failed to forward to an upstream Veneur")
 		}
 	} else {
 		entry.Info("Completed forward to an upstream Veneur")