enable get all metrics in CLUSTER_SERVERS

Enable:
"/metrics/index.json?cluster=True"
or
"/metrics/index.json?cluster=1"
styles to get all metrics in a cluster not only a single backend.

If "cluster=True" is provided, it will iterate all servers in this cluster to "GET /metrics/index.json", and merge all json response.


This function `urlopen` is not imported in the file.