[exporterhelper] Make the in-memory and persistent queues more consistent

[swiatekm]

Description:
This change is part of an effort to enable the persistent queue by default, without requiring the enable_unstable tag. Right now, there are minor inconsistencies between the queue behaviours, and this PR removes them. The intent is for enabling the persistent queue in the build by default to not change any behaviour.

More specifically:

• queue names are now consistent, which required a change to a metric label for the persistent queue
• the in-memory queue has the requeuing feature, though it's not enabled
• added some missing tests for persistent queue configuration

This was originally part of #5711, which ended up doing too many things at once.

Link to tracking Issue: N/A

Testing:
Added some tests for parts of the persistent queue that weren't originally covered.