Add a way to get the current run's metadata

[alaric-dotmesh]

As a python library user, I'd like a way to get the current run's proposed metadata, so that I can examine it for testing purposes.

ds.debug() comes close, but just prints it out. I'd like to have a dict.

ACs

• There is another method like ds.debug(), except that it returns a dict rather than printing JSON
• It has a good name (think of one)

[lukemarsden]

ds.debug() could just be changed to return the dict.