Unregister metrics when closing broker

Wanted to do this for a while, but upstream didn't have an unregister
method.

broker.go

@@ -178,6 +178,13 @@ func (b *Broker) Close() error {
 	b.done = nil
 	b.responses = nil
 
+	if b.id >= 0 {
+		b.conf.MetricRegistry.Unregister(getMetricNameForBroker("incoming-byte-rate", b))
+		b.conf.MetricRegistry.Unregister(getMetricNameForBroker("request-rate", b))
+		b.conf.MetricRegistry.Unregister(getMetricNameForBroker("outgoing-byte-rate", b))
+		b.conf.MetricRegistry.Unregister(getMetricNameForBroker("response-rate", b))
+	}
+
 	if err == nil {
 		Logger.Printf("Closed connection to broker %s\n", b.addr)
 	} else {

broker_test.go

@@ -77,10 +77,6 @@ func TestSimpleBrokerCommunication(t *testing.T) {
 			t.Fatal(err)
 		}
 		tt.runner(t, broker)
-		err = broker.Close()
-		if err != nil {
-			t.Error(err)
-		}
 		// Wait up to 500 ms for the remote broker to process the request and
 		// notify us about the metrics
 		timeout := 500 * time.Millisecond
@@ -91,6 +87,10 @@ func TestSimpleBrokerCommunication(t *testing.T) {
 			t.Errorf("No request received for: %s after waiting for %v", tt.name, timeout)
 		}
 		mb.Close()
+		err = broker.Close()
+		if err != nil {
+			t.Error(err)
+		}
 	}
 
 }