Enhance consul_serf_lan_member_info metric to include Leader role

[MaryTishchenkova]

Hello.

When using the metric consul_serf_lan_member_info, I would like to see the member's role to understand whether the member is a leader or not. Currently, the role only shows: role="consul" or role="node".

consul_serf_lan_member_info{instance="test1.example.com", job="consul", member="agent-test2-example-com", role="node", version="1.10.12"} 

consul_serf_lan_member_info{instance="test1.example.com", job="consul", member="server-test1-example-com", role="consul", version="1.10.12"}

It would be great if you could enhance this, as it would then provide the ability to track the leader node.