Add a docker-compose file to run BuildGrid with metrics enabled

Description

This MR adds another docker-compose file and some new config. This can be used to run a BuildGrid instance with monitoring enabled in the Execution service. The metrics are sent to Graphite using the statsd endpoint it provides, and a Grafana instance is also included at http://localhost:3000/ (default user/pass of admin/admin) with a basic dashboard pre-configured.