Expose `mergeRequestsCount` field in GraphQL Issue type
The REST API returns a merge_requests_count field for an issue. The GraphQL Issue type is missing an equivalent mergeRequestsCount field.
A mergeRequestsCount field should be added to the GraphQL Issue type to provide feature parity with the existing Haml issues page.
This merge request count is tested in https://gitlab.com/gitlab-org/gitlab/-/blob/master/spec/features/issuables/issuable_list_spec.rb#L59
Edited by Coung Ngo