Add prometheus metrics for origin save requests
Use a different approach compared to my first take on this in !997 (closed) (based on statsd).
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rely on prometheus Python client to generate and export some metrics related to origin save requests.
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add a new /metrics endpoint to the webapp in order for Prometheus to scrape metrics data.
A metric is generated for each of the available save request statuses: accepted, rejected, pending.
Related to T2124
Migrated from D2432 (view on Phabricator)