Insights Controller action for Average Issuables Per Author Per Status Per Period

This action aims to provide data to cover the following charts:

Charts

Average Issuables Per Author Per Status Per Period Combo Bar Line Chart

#	Name	Example
5	Avg Merged MRs Per Author Per Month	Link

These charts take:

• The number of Issuables with a particular status
• Group issuables by period merged
• Count unique authors for period
• Determine average based on number issuables and unique authors
• Also calculate rolling average based on previous 6 results => extracted to https://gitlab.com/gitlab-org/gitlab-ee/issues/11358.

5. Avg Merged MRs Per Author Per Month

• Issuable state: Merged
• Relevant timestamp: merged_at
• Relevant period: Months

Possible example yml

charts:
  - name: average_issuables_per_author
    title: Avg Merged MRs Per Author Per Month
    chart_type: combo_bar_line
    query:
      name: average_issuables_per_author
      params:
        issuable_state: 'Merged'
        timestamp: 'merged_at'
        period: 'months'