Metrics API: show number of user accounts

[stevenferey]

What this PR does / why we need it:

Closes issue #10177 following the pattern of existing metrics APIs:

http://$SERVER/api/info/metrics/$TYPE
http://$SERVER/api/info/metrics/$TYPE/pastDays/$DAYS
http://$SERVER/api/info/metrics/$TYPE/toMonth/$AAAA-MM
http://$SERVER/api/info/metrics/$TYPE/monthly

Which issue(s) this PR closes:

Closes #10177

Suggestions on how to test this:

curl -X GET -H 'Accept:application/json' http://$SERVER/api/info/metrics/accounts
curl -X GET -H 'Accept:application/json' http://$SERVER/api/info/metrics/accounts/pastDays/$DAYS
curl -X GET -H 'Accept:application/json' http://$SERVER/api/info/metrics/accounts/toMonth/$AAAA-MM
curl -X GET -H 'Accept:application/json' http://$SERVER/api/info/metrics/accounts/monthly
curl -X GET -H 'Accept:text/csv' http://$SERVER/api/info/metrics/accounts/monthly

Is there a release notes update needed for this change?:

Yes, the change is proposed with this file in PR:
doc/release-notes/10117-api-metrics-add-user-accounts-number.md

[pdurbin]

I'm excited to see this! Thanks, @stevenferey!

[coveralls]

coverage: 20.254% (-0.01%) from 20.267%
when pulling d9625c6 on Recherche-Data-Gouv:10177-api-metrics-add-user-accounts-number
into ba730cf on IQSS:develop.

[qqmyers]

Looks good. W.r.t. QA testing, it would be good to verify that the total accounts is correct and that all the endpoints work. Without installing on a long-running instance, it will be hard to check the time series, but the queries all follow the example from other endpoints.