metrics: fix memory leak

fixes #389

Credit goes to @sidenaio for finding this.

dht_net.go

@@ -114,7 +114,7 @@ func (dht *IpfsDHT) handleNewMessage(s network.Stream) bool {
 		timer.Reset(dhtStreamIdleTimeout)
 
 		startTime := time.Now()
-		ctx, _ = tag.New(
+		ctx, _ := tag.New(
 			ctx,
 			tag.Upsert(metrics.KeyMessageType, req.GetType().String()),
 		)

go.mod

@@ -27,3 +27,5 @@ require (
 	go.opencensus.io v0.21.0
 	golang.org/x/xerrors v0.0.0-20190513163551-3ee3066db522
 )
+
+go 1.12