Persist Prometheus after restart

[nodiesorg]

• What is the current behavior?
  Promethesus db and logs are not persisted after restart.

• If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem
  Run promethesus, then restart it a couple minutes later.

• What is the expected behavior?
  Metrics (timeseries db) should be persisted after restarting.

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow, gitter, etc)

@IanC mentions that your docker-compose template should persist

--storage.tsdb.path=/prometheus
volumes:
      - ./prometheus/:/etc/prometheus/

need to add the volume or move the tsdb.path