FFM-12192 patch 5

[erdirowlands]

What

Updates the keying mechanism for evaluation metrics to use a concatenated string format: "#{feature_name}\0#{variation_identifier}". This string combines the feature_name and variation_identifier separated by a null character (\0). This change replaces the previous approach of keying on the MetricsEvent class, which has now been deleted.

Why

We see an edge case occurring during the iteration of evaluation metrics when aggregating and sending data. Specifically, the keys seem to reference other objects in memory, and seg faults. Although Ruby permits using class objects as keys, the evaluation cache uses a hash with string keys and does not suffer from the same problem, so the intent is standardise on string keys to see if we can fix this edge case.

Testing

• Updated unit tests to reflect the new key structure
• Manual tests with Prod2 app
• Testgrid

[davejohnston]

This change looks good