Enable Gitaly Prometheus Listener
Gitaly has been shipped as part of 9.0 but the prometheus listener is disabled by default.
According to https://github.com/prometheus/prometheus/wiki/Default-port-allocations, Gitaly should expose Prometheus metrics on port 9236, and therefore the Gitaly service needs the following environment variable to be set in order to do this:
GITALY_PROMETHEUS_LISTEN_ADDR = ":9236"
Note the colon before the port number.