Add a custom metric to prometheus. (#153)

This works! We can use it to track # of new entries over time.

pkg/api/entries.go

@@ -108,6 +108,9 @@ func CreateLogEntryHandler(params entries.CreateLogEntryParams) middleware.Respo
 		}
 	}
 
+	// We made it this far, that means the entry was successfully added.
+	metricNewEntries.Inc()
+
 	queuedLeaf := resp.getAddResult.QueuedLeaf.Leaf
 	uuid := hex.EncodeToString(queuedLeaf.GetMerkleLeafHash())
 

pkg/api/metrics.go

+package api
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+var (
+	metricNewEntries = promauto.NewCounter(prometheus.CounterOpts{
+		Name: "rekor_new_entries",
+		Help: "The total number of new log entries",
+	})
+)