Extend Insights to support the query option

Release notes

This issue is a breakdown from a spike issue which makes the DORA metrics exposable via the Insights feature.

High-level Steps

• Introduce an extra library layer for supporting the new YAML schema and delegating to the correct backend (finder).
• Adjust the Insights parameter validation rules to accommodate the new schema changes.
• Update the documentation with examples.

Details

To make this work, we can introduce an extra layer (maybe a lib?) which is responsible for the YAML options processing and delegation to the correct backend (finder). This is currently implemented as a mixin on the controller-level: InsightsActions

After this change, the controller would look much cleaner, something like this:

def query
  Gitlab::Insights::Validators::ParamsValidator.new(params).validate!

  render json: Serializer.new(Insights::Loader.new(chart_params).call)
end

The Loader is responsible for the following:

• Handle/transform the incoming parameters in a backwards-compatible manner (old Insights files should still work.
• Delegate to the correct Finder class and return the data to the serializer.

Configuration

Old configuration (it should work with the new changes):

bugsCharts:
  title: "Charts for bugs"
  charts:
    - title: "Monthly bugs created"
      description: "Open bugs created per month"
      type: bar
      query:
        issuable_type: issue
        issuable_state: opened
        group_by: month
        period_limit: 12

New configuration (with 2 charts, Issuable finder based and DORA base):

bugsCharts:
  title: "Charts for bugs"
  charts:
    - title: "Monthly bugs created"
      description: "Open bugs created per month"
      type: bar
      query:
        data_source: issuable
        params:
          issuable_type: issue
          issuable_state: opened
          group_by: month
          period_limit: 12
    - title: "DORA DF"
      description: "Deployment frequency"
      type: bar
      query:
        data_source: dora
        params:
          metric: deployment_frequency
          environment_tiers: 
            - production
          period_limit: 12
  

Note: the DORA finder might not support all the possible chart times or period options.