Prometheus exporter with a few metrics

The goal of the PR is to introduce some Prometheus metrics about the app that will help better track its state. It currently exposes some basic python metrics and three custom ones:

Sample (without comments):

margebot_info{version="0.14.0"} 1.0
margebot_current_mr{author_id="7169209",iid="4977",title="Docs: link long tezts Grafana dashboard and logs",web_url="https://gitlab.com/tezos/tezos/-/merge_requests/4977"} 4977.0
margebot_pending_mr_total{pending="4977"} 1.0