Add listener ID tag to UDS-specific metrics in DogStatsD source.

[tobz]

Context

Title says it all, really.

We effectively want to match the behavior shown here in the Datadog Agent.

[tobz]

I'm going to close this for now as "won't do."

In the Datadog Agent, the listener ID, by default, ends up having a value of uds-unix or uds-unixgram, which is essentially identical to the existing transport tag on DogStatsD listener-specific metrics.

The listener ID only gets more useful with "extended" support on, where it throws in the file descriptor of the remote peer as part of the ID, so like uds-unixgram-42 or whatever. Since that's meant purely for very advanced debugging, we're going to punt on this, overall, for now.