`metric_id` and `metricId` duplicate data in metrics, remove one of them

metric_id and metricId duplicate data in metrics. We should remove one of them.

Notes

It might make sense to keep metric_id as the whole dashboard data structure will be POSTed back to the backend at some point to allow the user to modify the dashboard.

Original discussion

The following discussion from !20621 (merged) should be addressed:

• @psimyn started a discussion:

  is there an issue to get replace usages of metric_id?