We don't want to be setting statsd_namespace--that's not our standard
for metrics. Ergo, we should remove it.

This captures the metrics used in the tests in a `metrics_emitted.txt`
file. You can uniquify the contents of this file to determine what got
emitted and compare it from run to run.

This changes HOST_ID to HOSTNAME to match the convention and what we get
in GCP.

If the hostname is set, this will add a "host:{hostname}" tag to all
emitted metrics.