Move the object counter from munin to prometheus

The only munin metric that an external tool depends on is the object counter[6], which is exported from munin to the main website to make the graphs. The object counter needs to be moved to prometheus, and the export pipeline needs to be rewritten to use a prometheus query. This should be fairly trivial as we already use the prometheus sql exporter for other metrics[7,8].

[6] http://munin.internal.softwareheritage.org/softwareheritage.org/prado.softwareheritage.org/softwareheritage_objects_softwareheritage.html [7] https://github.com/credativ/sql_exporter [8] https://forge.softwareheritage.org/source/puppet-swh-site/browse/production/data/hostname/prado.softwareheritage.org.yaml$8-33

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Migrated from T1355 (view on Phabricator)