[#290] Resolve "add basic APM monitoring to signald"

Closes #290 (closed)

context

• we want to start investigating message delivery lag under load by instrumenting signald with prometheus metrics exporting capabilites
• the goal of this MR is to get the ball rolling by measuring the simplest valuable thing: what proportion of time in Manager.sendMessage is spent in network calls vs. file system operations

implementation details

use signald build with prometheus instrumentation:

• repo: https://0xacab.org/team-friendo/signald-fork.git
• branch: master
• commit: 0ece4970980a5f4d768339fbe067baa6cc6f62a0

expose signald metrics exporting server from same box as signalboost app by exttracting signalboost/signald host/port into env vars and injecting them into docker configs

• context: we want to provide different VIRTUAL_HOST and VIRTUAL_PORT env vars to signalboost and signald containers so that our ngix container can proxy traffic to each app container correctly based on hostname and port on incoming requests
• solution: declare SIGNALBOST_HOST_URL, SIGNALBOOST_PORT, SIGNALD_HOST_URL, and SIGNALS_PORT in .env, read them into the appropriate containers in docker-compose files

restrict access to signald metrics server by whitelisting prometheus server IP in signald ngix configs

• context: we want to expose an http server from the signald container without doing too much authentication work (in part b/c the built in server doesn't support inspecting headers, etc. and we don't want to bother writing a jetty server just for this one endpoitn)
• solution:
  • expose an unauthenticated endpoint (only accessed over TLS) in the signald container and map traffic from signald.signalboost.info to this container via the nginx proxy container
  • whitelist localhost and the prometheus server's IP address as the only address that can access signald.signalboost.info via the vhost.d file for the signald.signalboost.info config file injected into our nginx proxy
• credentials book-keeping:
  • registered in blackbox: nginx/vhost.d/signald.signalboost.info (to accomplish IP whitelisting)
  • edited in blackbox: prometheus/prometheus_dev.yml, prometheus_prod.yml (to add signald scrape job configs)