The following has been broken down into three subtasks:

• validate the cohort by username, not failing unless there are more than 50% invalid names
• run a few different reports with this cohort (but the specific reports that run can be a hard-coded configuration, we're not expecting this to be a common thing we need to build generic infrastructure for if that's too hard.
• get the results from the reports
• delete the cohort (or just never create it if that's easier, but I think MultiProjectMetricReportNode needs it)
• put the results in the db with a guid id defined by the API when it first accepted the request

Additional work:

• add bytes added metric to output
• pages improved, maybe resurrect this: https://gerrit.wikimedia.org/r/#/c/174773/1/wikimetrics/metrics/pages_edited.py
• fix cohort size logic