Emit metrics for local memberlist size and remote memberlist size

[jmurret]

Changes proposed

• emit a "memberlist", "node", "instances" Gauge and pass label so a gauge can be broken out by node state
  • it appears gauges are reported with the host/pod name in the metric. so the above gauge will emit metrics in the sink (grafana) for 1 server and 3 clients as:
    • consul_server_0_memberlist_node_instances
    • consul_client_1_memberlist_node_instances
    • consul_client_2_memberlist_node_instances
    • consul_client_3_memberlist_node_instances

• emit "memberlist", "size", "local" Gauge that reports the size in bytes of the memberlist size before it is sent to another gossip recipient.
  • same condition as above for multiple metrics being create by hostname

- emit `"memberlist", "size", "remote"` Gauge that reports the size of incoming memberlists from other gossip senders - is this helpful? I feel like we would have to add a label for who is sending it?

[jmurret]

suggestions for a better metric name? (also, see question below about removing the remote metric, which might make naming this easier.)

[rboyer]

Once the library versioning bumps are done these can be documented on the consul telemetry page on the website.