Metrics should only be exposed via the /bridges/:id/metrics endpoint,
and not in other operations such as getting the list of all bridges, or
in the response when a bridge has been created. This commit removes all
traces of metrics for the non-dedicated API endpoints.
In order for the /bridges APIs to be consistent with other APIs, we move
out metrics from GET /bridges/{id} to its own endpoint,
/bridges/{id}/metrics. We also rename /bridges/reset_metrics to
/bridges/metrics/reset.
Erik Timan