CopyFromRoot config option

[tdarwin]

Is your feature request related to a problem? Please describe.

Baggage is painful and fraught with risks if you don't properly lock down your code to not send it to 3rd parties. But a lot of customers still want to copy fields from root spans to the rest of the trace.

Describe the solution you'd like

A config option where you can say you'd like to copy fields from the root span, what those attribute names are, and what the prefix is that you'd like to use when copying those attributes to other spans.

When the root span comes in, grab the available field values and add those with their prefixes to the spans in the trace at the same point we add things like the meta.refinery attributes, then send the spans on.

Describe alternatives you've considered

Not a good way to do this in the collector today, but Refinery is already doing the hard part of being trace aware, and is already updating all the spans with various extra fields, so it seems like this won't add a lot of overhead, while still opening the door to "refine" data as well as just sample data.

Additional context

Have a customer that built and tested some code that did this, though I imagine with 2.8 and 2.9 this is a bit more complicated as we get rid of trace locality, and likely adds a bit of pub/sub traffic to Redis, but feels like it could be super cool to have.

[TylerHelmuth]

Implementation PoC here: https://github.com/honeycombio/refinery/tree/prop-root-span-fields