Add prometheus endpoint for getting metrics (#33)

vendor/github.com/go-redis/redis/cluster_commands.go

@@ -0,0 +1,22 @@
+package redis
+
+import "sync/atomic"
+
+func (c *ClusterClient) DBSize() *IntCmd {
+	cmd := NewIntCmd("dbsize")
+	var size int64
+	err := c.ForEachMaster(func(master *Client) error {
+		n, err := master.DBSize().Result()
+		if err != nil {
+			return err
+		}
+		atomic.AddInt64(&size, n)
+		return nil
+	})
+	if err != nil {
+		cmd.setErr(err)
+		return cmd
+	}
+	cmd.val = size
+	return cmd
+}