Descriptions of some vBucket stats are missing

I can't find these stats described anywhere in our documentation. Please provide clear descriptions for each with examples relevant to Couchbase buckets and Ephemeral buckets.

• avg_active_timestamp_drift
• avg_replica_timestamp_drift
• active_ahead_exception
• replica_ahead_exception

I have customers who have experimented with these and arrived at their own understanding of what these are. These customers are interested in using these stats but are holding back due to lack of "official documentation".