Add `watermark: date` to stats response here https://github.com/10gen/mongo-enterprise-modules/blob/master/src/streams/commands/stream_ops.idl#L153.

See some existing test cases in stats.js: https://github.com/10gen/mongo-enterprise-modules/blob/master/jstests/streams/stats.js

Information in the design doc about what watermarks are:

https://docs.google.com/document/d/10wktWq3u8jRALXR0EUb23_3ZSmv6AzqGTf0zXqVIvms/edit#heading=h.u26ofm59d6h0