metric to track batch retries

beacon-chain/sync/backfill/batch.go

@@ -152,6 +152,7 @@ func (b batch) withState(s batchState) batch {
 		b.scheduled = time.Now()
 		switch b.state {
 		case batchErrRetryable:
+			backfillBatchRetries.Inc()
 			b.retries += 1
 			log.WithFields(b.logFields()).Info("Sequencing batch for retry")
 		case batchInit, batchNil:

beacon-chain/sync/backfill/metrics.go

@@ -57,6 +57,12 @@ var (
 			Help: "Number of BeaconBlock values downloaded from peers for backfill.",
 		},
 	)
+	backfillBatchRetries = promauto.NewCounter(
+		prometheus.CounterOpts{
+			Name: "backfill_batch_retries",
+			Help: "Number of times batches have failed with a retryable error.",
+		},
+	)
 	backfillBatchTimeRoundtrip = promauto.NewHistogram(
 		prometheus.HistogramOpts{
 			Name:    "backfill_batch_time_roundtrip",