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. We also add an enum representing the possible metrics, to avoid similar future mistakes.
Closes#7520Closes#7523
Files were generated with the `thor api:generate:spec --force --verbose --crawl` command,
and based on the 0.90 branch (elasticsearch/elasticsearch@5ec4064).