Extend API to allow the addition of attributes to unclosed-but-created spans

[ELLIOTTCABLE]

At the moment, the ~data API requires all attributes to be known at span-creation time. This isn't always possible.

Consider the case of an HTTP request, on the client-side — when the span is opened, one cannot know the response-code of that request; but before the span is entirely closed, you'd want to attach that information to the span.

Something like Trace.add_data ~data span_id, perhaps?

[c-cube]

Yes indeed, that's a very reasonable feature. 👍