Implement Covered Experience event emission in labkit-ruby

The attributes are specified in the blueprint section covering the SDK requirements.

Attributes such as correlation_id, feature_category, type, and meta will be always be pulled from the current labkit context, while covered_experience_id, checkpoint, and checkpoint_category will vary depending on the event being emitted -- either pushing a new value to the context (start event), as pulling from the context (checkpoint, end events).

• design covered experience public API
• emit metric events
• log events