Feature: Implemented prometheus metrics

[flymia]

This PR fixes #77. It adds Prometheus metrics, which show instance wide stats which can be used to visualize everything on Grafana.

Example output:

# HELP yaams_aircraft_total Total number of aircraft by lifecycle status
# TYPE yaams_aircraft_total gauge
yaams_aircraft_total{status="active"} 2
yaams_aircraft_total{status="inactive"} 0
yaams_aircraft_total{status="retired"} 0
# HELP yaams_airlines_total Total number of airlines
# TYPE yaams_airlines_total gauge
yaams_airlines_total 2
# HELP yaams_flights_total Total number of filed flights (PIREPs) by review status
# TYPE yaams_flights_total gauge
yaams_flights_total{status="accepted"} 1
yaams_flights_total{status="pending"} 0
yaams_flights_total{status="rejected"} 0
# HELP yaams_invite_codes_unused_total Number of generated invite codes not yet redeemed
# TYPE yaams_invite_codes_unused_total gauge
yaams_invite_codes_unused_total 0
# HELP yaams_queue_failed_jobs_total Number of jobs in the failed_jobs table
# TYPE yaams_queue_failed_jobs_total gauge
yaams_queue_failed_jobs_total 0
# HELP yaams_queue_size The total number of jobs in the queue
# TYPE yaams_queue_size gauge
yaams_queue_size{connection="database",queue="default"} 0
# HELP yaams_users_total Total number of registered users
# TYPE yaams_users_total gauge
yaams_users_total 2

To configure it, you need to add the METRICS_TOKEN to the .env file.