Add Metrics API to get datasets by subject AND to month

[scolapasta]

Currently, we can get datasets up to a certain month (and determine how many new by subtracting the previous month) or by subject, but one of the desired visualizations is to get by subject for a specific month, i.e. am metric to get to a certain month AND by subject.

[pdurbin]

After standup today, @landreev and I discussed that while 0.1 of the metrics aggregator can handle the 6 existing metrics endpoints, it should be updated to handle the new 7th endpoint that's being created.

Here's the current config file for the metrics aggregator with the 6 endpoints: https://github.com/IQSS/metrics.dataverse.org/blob/v0.1.0/config.json.sample

[landreev]

Made a pull request (#5451); this should not go into CR just yet - as I'm adding this new metrics to the aggregator in the other code repository, as described above. (thanks @pdurbin)