Fix memory leak in loki.process on config update

[ptodev]

PR Description

A goroutine wasn't being stopped in Update, which resulted in a memory leak.

The same change was also introduced to Alloy.

PR Checklist

• [x] CHANGELOG.md updated
• [ ] Documentation added
• [x] Tests updated
• [ ] Config converters updated

[ptodev]

Hi @thampiotr, would you mind reviewing the latest commit please? The unit tests kept being flaky, and I realised it's because the loki.process code needs to be improved further to shut down more gracefully. If you're happy with this change, I'll also open a PR for Alloy to have it.