Issues Analytics: Add total number of open issues total chart (backend)

Problem to solve

Currently Issues Analytics only shows open issues. As explained in #233905 (closed), it would be valuable if it also showed total cumulative opened issues.

Proposal

We would need to create a new endpoint, or extend the analytics_issues-endpoint (I cannot find an existing one) to return total_issues for a given month, this requires several count queries to the database with potentially no limit since we need to count "issues created before a certain date and issues either not closed at all or closed after a certain date" for each month. (source)

Total open issues = number of issues opened before the end of given month and (not closed at all OR closed after the end of given month).

Technical details

We can do 1 query for all opened issues before selected timespan and then do the math for selected period with small queries:

total_jan = (created_at <= Jan AND closed_at IS NULL OR closed_at > Jan)
total_feb = total_jan + opened_feb - closed_feb
total_mar = total_feb + opened_mar - closed_mar
....