This change adds the two following change stream specific throughput metrics on mongoS:

• *metrics.changeStreams.cursor.bytesRead* (counter): Shows the
  cumulative amount of scanned oplog in bytes by all change stream
  cursors.
• *metrics.changeStreams.cursor.docsExamined* (counter): Shows the
  cumulative number of scanned oplog entries by all change stream cursors.

Description of Linked Ticket

Add change stream specific throughput metrics on mongoS (see project design document section "Decision: Process-Level Change Stream Cursor Metrics / Throughput"):

• metrics.changeStreams.cursor.docsExamined
• metrics.changeStreams.cursor.bytesRead

Design notes:

• The docsExamined and bytesRead should be aggregated from shards in AsyncResultsMerger (similar to what we already do for $queryStats)