Add creditsPerMergedMrRatio metric to the duoWorkflows source

Registers creditsPerMergedMrRatio as a flat metric on the duoWorkflows source, so GLQL queries can request credits consumed per merged MR directly. This backs the "Cost per merged change" tile on the DAP Impact Overview tab, gitlab issue gitlab-org/gitlab#630202.

Blocked on the gitlab engine MR (gitlab!256889 (merged)) reaching master: CI validates compiled queries against the gitlab GraphQL schema, and the new field only exists there once that MR merges. After it merges, this needs a glql release plus a version bump in gitlab before the dashboard tile MR (gitlab!256890 (merged)) can go ready.

Full cargo test suite is green, including the new duo_workflows test. fmt and clippy are clean.