metamorphic: report db metrics after run

Have the metamorphic tests report metrics after each run to get a better picture of what was exercised during a metamorphic test.
Ideally, we can also aggregate metrics across stress runs.

Jira issue: PEBBLE-1096

Start by locating the metamorphic test runner and the path used for stress runs. Inspect how database metrics are exposed after a run, then define reporting for each run and aggregation across stress runs. Done means metamorphic tests provide per-run metrics and combined stress-run metrics.

Written by the indexing model from the issue text.