fixes after @xla review

- move prometheus metrics into internal packages
- *Option structs
- misc. format changes

mempool/metrics.go

@@ -3,6 +3,9 @@ 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.
@@ -12,6 +15,17 @@ type Metrics struct {
 	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{