feat: add session labels on connection metrics

[bavarianbidi]

to differenciate multiple running upterm host sessions via the metrics endpoint, new metrics will get the session_id label and a custom label.

This is an initial implementation to get a technical breakthrough to see how/if the desired feature request #194 could look like.

The current implementation will generate the following new metrics:

# HELP uptermd_upterm_host_current_clients Info about started upterm host session
# TYPE uptermd_upterm_host_current_clients gauge
uptermd_upterm_host_current_clients{controller="upterm",session_id="6Xugtn2fpoFEAxEXTkLr"} 1
uptermd_upterm_host_current_clients{controller="upterm",session_id="lcMJta3WHPyOhGxbWClH"} 2
# HELP uptermd_upterm_host_info Info about started upterm host session
# TYPE uptermd_upterm_host_info gauge
uptermd_upterm_host_info{controller="upterm",label="this-is-my-custom/hostname",session_id="6Xugtn2fpoFEAxEXTkLr"} 1
uptermd_upterm_host_info{controller="upterm",label="clx082456336.company.domain.net",session_id="lcMJta3WHPyOhGxbWClH"} 1

following upterm host commands are run to generate the metrics:

• don't define a custom label*
  the os.getHostname will be used as custom label

make build && ./build/upterm host --accept --server ssh://localhost:2222 -- docker run --rm -ti ubuntu bash

• overwrite the discovered hostname*

make build && ./build/upterm host --accept --server ssh://localhost:2222 --label "this-is-my-custom/hostname" -- docker run --rm -ti ubuntu bash

To be discussed:

• Is this something you might accept @owenthereal
• define better metric names
• how to deal with the already existing metrics? should we re-use upterm_uptermd_routing_active_connections_count and
  upterm_uptermd_routing_connections_count

[bavarianbidi]

@owenthereal friendly reminder on my questions in this PR ;-)

i'm also happy to chat/explain our current usage of upterm with you.
Maybe that might help to at least understand the origin of some of our PRs/Issues.