Make a /metrics handle at the snowflake broker for the stats collector

We have metrics being written to a log file for now as specified in legacy/trac#21315 (moved), and the metrics team is working on setting up a collector module for it in legacy/trac#29461 (moved). We should add a handler at the broker at snowflake-broker@bamsoftware.com/metrics so the collector can make GET requests for the collected metrics.

The response should be Content-Type: text/plain; charset=utf-8