Pass related_class to the context as a string

What does this MR do?

Since the Gitlab::ApplicationContext wants a string, this means that anything else will not be passed to Labkit for logging.

We did this to avoid logging huge objects that could contain sensitive information.

This also caused the ReactiveCachingWorker to miss it's related class sometimes (gitlab-com/gl-infra/scalability#258 (closed)): We schedule this in `ReactiveCaching#refresh_reactive_cache! using self.class, this would then filtered out since Class is not a String.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done