Control which instance exports cluster metrics

[Neraud]

Currently, cluster metrics are only exported on the Leader node.

For a GD1 cluster, that's the peer with the maximum UUID (lexicographically) (https://github.com/gluster/gluster-prometheus/blob/master/pkg/glusterutils/exporterd.go#L23)

In my case, that happens to be a smaller machine than other peers, and running the exporter with all collectors enabled generates a visible load (and audible fan speed increase).
(TBH, my home cluster is using small hosts and that's most probably not a recommended sizing).

It would be great to be able to control on which host cluster metrics are exported.

For example, using a configuration flag : expose-cluster-metrics: [auto|true|false]
auto would be the default and would still use the current IsLeader test.
true and false would override the IsLeader test.