Follow-up to SERVER-122364.

For mongoS we must aggregate the following lag-related metrics at the cursor-level retrieved from shards:

• "changeStreams.cursor.optime"
  • The optime of the latest event that we've sent or the current 'high-watermark'.
• "oplog.earliestTime"
  • The smallest optime across all shard oplogs.
• "oplog.latestTime"
  • The largest optime across all shard oplogs.

An important implementation decision is when to update the metrics, since some of the oplog scanning functionality is done by the classic find-subsystem. A viable option is to do this in the earliest change stream stage of the aggregation pipeline ($_internalChangeStreamOplogMatch).