Trim return value of prometheusHttpPrefix and alertManagerHttpPrefix

[lamida]

What this PR does

Which issue(s) this PR fixes or relates to

Fixes #3086

Checklist

• [ ] Tests updated
• [ ] Documentation added
• [x] CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[lamida]

Thanks for fixing this! Should we add a changelog entry too?

Yes, this is still a draft. Will add everything required. :)

[pracucci]

The CHANGELOG has just been cut to prepare for the next Mimir release. Please rebase main and eventually move the CHANGELOG entry added / updated in this PR to the top of the CHANGELOG document. Thanks!

[lamida]

Please rebase as 3.2.0 changelog was cut already, otherwise ok

@krajorama reabase is done.