Add data stream name to document metadata

[billfnt]

Description

Greetings,

Just like we have the _index metadata field that lets us know what index a document is in, it would be handy to have a metadata field to let us know what data stream the document is in. The intended use case here is to create visualizations showing relative size (document count and _size values) for the various datastreams in our cluster as well as track the datastream volumes over time.

Thanks,

-Bill.

[elasticsearchmachine]

Pinging @elastic/es-data-management (Team:Data Management)