Metadata fields on stream and error outputs

[rgbkrk]

I'd like to support metadata fields on all the output types. The ones that can't currently are stream and error.

/cc @craigcitro, who I talked to about this yesterday

[blink1073]

👍

[craigcitro]

Yes, this would be really helpful for us, too. (More helpful than my slow reply would suggest!)

In particular, in Colaboratory, we use metadata on stream outputs, in order to do various forms of output control. We actually didn't realize at first that this was invalid, so we do some gyrations to always ensure this metadata is dropped when writing notebooks out to disk.

[rgbkrk]

Thinking on this one after a nice week with the Jupyter devs, how would we merge metadata (if merging streams, like is done in the notebook).

[rgbkrk]

@craigcitro -- how does metadata get combined if multiple stdout messages come in (that can be "merged" into one, since it's a terminal representation).

[craigcitro]

At least for colab, we only merge if the messages have identical metadata.