REST: Reroute API response didn't filter metadata

By default the reroute API should return the new cluster state, excluding the metadata. It was however it was wrongly using an old parameter (filter_metadata) and thus failed to do so. This commits restores but wiring it to the correct `metric` parameter.

Closes #7520 

PS. The name `metric` for filtering cluster state feels wrong, but this remains for another change (and a discussion)