Revert "delete everything" (includes everything non-go-crypto)

This reverts commit 96a3502

mempool/metrics.go

@@ -0,0 +1,34 @@
+package mempool
+
+import (
+	"github.com/go-kit/kit/metrics"
+	"github.com/go-kit/kit/metrics/discard"
+
+	prometheus "github.com/go-kit/kit/metrics/prometheus"
+	stdprometheus "github.com/prometheus/client_golang/prometheus"
+)
+
+// Metrics contains metrics exposed by this package.
+// see MetricsProvider for descriptions.
+type Metrics struct {
+	// Size of the mempool.
+	Size metrics.Gauge
+}
+
+// PrometheusMetrics returns Metrics build using Prometheus client library.
+func PrometheusMetrics() *Metrics {
+	return &Metrics{
+		Size: prometheus.NewGaugeFrom(stdprometheus.GaugeOpts{
+			Subsystem: "mempool",
+			Name:      "size",
+			Help:      "Size of the mempool (number of uncommitted transactions).",
+		}, []string{}),
+	}
+}
+
+// NopMetrics returns no-op Metrics.
+func NopMetrics() *Metrics {
+	return &Metrics{
+		Size: discard.NewGauge(),
+	}
+}