Grant execute on metric_chunks_that_need_to_be_compressed to prom_reader

[jgpruitt]

Description

The connector uses this function to expose a prometheus metric and that part of the code is restricted to the prom_reader role.

Merge requirements

Please take into account the following non-code changes that you may need to make with your PR:

• [ ] CHANGELOG entry for user-facing changes
• [ ] Updated the relevant documentation