The route for `get_runs_by_run_metadata_key` conflicted with the
`get_test_runs_by_build_name` route when grouping by the
`build_name` metadata key. This moves the route into the
`/runs/key/` namespace to prevent it from colliding with other
endpoints.
Change-Id: I64280febe39eaecd7b694aea7f5d39df5c1bc45c
821601b16d