Supported values are:

- `created_at` (default): Group data using the `created_at` field.
- `closed_at`: Group data using the `closed_at` field (for issues only).
- `merged_at`: Group data using the `merged_at` field (for merge
  requests only).

The `period_field` is automatically set to:

- `closed_at` if `query.issuable_state` is `closed`
- `merged_at` if `query.issuable_state` is `merged`
- `created_at` otherwise
Signed-off-by: Rémy Coutable <remy@rymai.me>