add more prometheus metrics

This exposes some relevant prometheus metrics at /metrics. Available metrics include:

total number of parent maintenances - that is, maintenances with a maintenance_id
total number of maintenances on a circuit. This is separated from parent maintenances because one parent maintenance may affect the same circuit over the course of several maintenance window under a single maintenance id
maintenances in progress

This requires the gunicorn_config.py file to be added as gunicorn config. For example:

gunicorn -b localhost:5000 -w 4 -c gunicorn_config.py janitor:app --preload

This addresses #1

wasabi222 / janitor