feat(metrics): Add remote IP label to containerssh_ssh_current_connections

[gaocegege]

Please describe what you would like to see in ContainerSSH

The remote server IP in ssh-proxy backend may change with the config webhook. We cannot get the current connections for a specific server. Thus I propose to add a new label server-ip to the metric containerssh_ssh_current_connections

Please describe your use case

I will create multiple ssh backends. And use them in ssh-proxy mode. The server IP is updated by the config server webhook. And I wanna know how many connections are alive in the server.

This may have to be an optionally enabled feature since the cardinality can explode if many different backends are used.

[gaocegege]

SGTM.