Kv uses ids instead of names when reporting collection metrics to prometheus

Kv uses ids in collection and scope labels instead of their names when reporting collection metrics to prometheus:

kv_items\{collection="0x0",bucket="test",scope="0x0"} 0.000000

kv_items\{collection="0x8",bucket="test",scope="0x0"} 1.000000

First of all, it's inconsistent with buckets.

Other infrastructure (such as ns_server's API's) also assumes to see names as collection and scope label values.

In future we might want to add id's as a separate labels in order to deal with bucket/scope/collection deletion, but strategy for this use-case is not defined yet. Right now we need names in collection metrics for MVP.