Observe feature flags

What does this MR do and why?

This MR will allow feature flags to be tracked within Kibana so that we can segment requests when the feature flag was enabled vs disabled. This will help us roll out large and complex changes more safely.

Observed feature flags have been limited to a maximum of 10 feature flags to avoid cardinality concerns. I thought about adding something like an expiration but did not want to complicate the MR. The long term fix is to remove limits while retaining the functionality.

There is further documentation within doc/development/feature_flags/_index.md

Related to #582272

References

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