Add more client stats

[stevej]

Debugging the mystery of the failing iperf tests would have been easier had /metrics shown me that there were other clients connected and where they were connected from.

[olix0r]

We probably shouldn't expose any per-endpoint metrics by default unless we have a good way of garbage collecting unused enpdpoints (i.e. we'd need to ensure that dropping an endpoint removes its counters, but only after they've been reported... it's a bit tricky).

What non-per-endpoint metrics would have helped you?

[stevej]

non-per-endpoint metric: Unique number of currently connected clients.

I would have expected 1 when running my load test with a concurrency of 1. Seeing > 1 would have led me to login to the host and use ss to see who else was connected.

[tamird]

@olix0r this should have been closed by ea507f7.

FYI this doesn't do what you think:

Fixes #1 #2 #3"

Each issue needs its own "fixes":

Fixes #1
Fixes #2
Fixes #3