Report mempool size in metrics

bchain/coins/blockchain.go

@@ -153,7 +153,11 @@ func (c *blockChainWithMetrics) SendRawTransaction(tx string) (v string, err err
 
 func (c *blockChainWithMetrics) ResyncMempool(onNewTxAddr func(txid string, addr string)) (count int, err error) {
 	defer func(s time.Time) { c.observeRPCLatency("ResyncMempool", s, err) }(time.Now())
-	return c.b.ResyncMempool(onNewTxAddr)
+	count, err = c.b.ResyncMempool(onNewTxAddr)
+	if err == nil {
+		c.m.MempoolSize.Set(float64(count))
+	}
+	return count, err
 }
 
 func (c *blockChainWithMetrics) GetMempoolTransactions(address string) (v []string, err error) {

common/metrics.go

@@ -18,6 +18,7 @@ type Metrics struct {
 	IndexResyncErrors     *prometheus.CounterVec
 	IndexDBSize           prometheus.Gauge
 	ExplorerViews         *prometheus.CounterVec
+	MempoolSize           prometheus.Gauge
 }
 
 type Labels = prometheus.Labels
@@ -113,6 +114,13 @@ func GetMetrics(coin string) (*Metrics, error) {
 		},
 		[]string{"action"},
 	)
+	metrics.MempoolSize = prometheus.NewGauge(
+		prometheus.GaugeOpts{
+			Name:        "blockbook_mempool_size",
+			Help:        "Mempool size (number of transactions)",
+			ConstLabels: Labels{"coin": coin},
+		},
+	)
 
 	v := reflect.ValueOf(metrics)
 	for i := 0; i < v.NumField(); i++ {