[Prometheus] /metrics endpoint doesn't return KV metrics

Summary:
/metrics endpoint doesn't return KV's metrics (both low and high cardinality metrics)

Steps to reproduce
1. Create a 1 node cluster (172.23.120.137) (kv:index:query)
2.

curl -v -u Administrator:password http://172.23.120.137:8091/metrics

returns ns_server, index, and query metrics, but doesn't return KV's metrics 
(although exposition of KV metrics to prometheus works fine, and stats are scraped)

Discussed with Timofey on Slack before filing  CC: Timofey Barmin

Output from second step
curl_output.txt