Expose DORA 4 data through GraphQL

Background

We've recently added API support for two DORA 4 metrics: deployment frequency and lead time: https://docs.gitlab.com/ee/api/dora/metrics.html

Release notes

Users should be able to programmatically access DORA 4 data through both the REST API and the GraphQL endpoint.

This feature is GitLab Ultimate.

Proposal

Expose the DORA 4 data (already available through the REST API) through the GraphQL endpoint.