Is your feature request related to a problem? Please describe.
Nylas API allows for setting a metadata field on Calendar, Event, Message, and Draft objects, but this repo doesn't currently account for this (documentation). It would be nice to be able to add metadata when sending Messages specifically (can see the API allows it on requests here).

Describe the solution you'd like
It seems reasonable to add a metadata field onto the SendMessageRequest model (and potentially the other "Send" models that metadata applies to).

Describe alternatives you've considered
Don't think there are any workarounds here.

Additional context
Would also be nice if Nylas API could include the metadata field on message.created webhooks -- it looks like they currently include all other Message fields except the metadata which is pretty inconvenient since it forces consumers to have to make an additional http call to the GET message endpoint to see the metadata.