swh-search: add statsd/prometheus metrics

There is no metrics generated by the swh-search service so we can't graph any performance statistics currently.

The only stats at the moment are on the gunicorn logs :

Feb 12 11:16:47 moma python3[2477829]: 2021-02-12 11:16:47 [2477829] elasticsearch:INFO GET http://search-esnode2.internal.softwareheritage.org:9200/origin/_search?size=100 [status:200 request:0.722s]

---

Migrated from T3042 (view on Phabricator)