Improve speed of query used in available_metrics

[IvanIvanoff]

Changes

The improved query is one of the queries used to compute available_slugs. If it is too slow, the whole computation of available_slugs fails. available_slugs is cached in the RehydratingCache which will then start to try to compute it over and over again (for all queried slugs), which puts a lot of pressure on Clickhouse, slowing all the API queries.

Ticket

Checklist:

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have tried to find clearer solution before commenting hard-to-understand parts of code
• I have added tests that prove my fix is effective or that my feature works