```
etcd_server_id{server_id="8e9e05c52164694d"} 1
```
Useful for automating membership change operations,
no need to run "endpoint status" or "member list"
command to get member IDs.
Also, useful for "etcd_network" metrics with "To/From" labels.
Signed-off-by: Gyuho Lee <leegyuho@amazon.com>