Capture request_id in traces

[dbolduc]

@coryan points out that

• We auto populate request_id-like fields in the connection layer.
• This is valuable information for debugging
• So we should include it in our traces.

He proposes that:

• we should include the attribute even if the value is empty, because that is a bug and thus "interesting".
• we name the attribute: gl-cpp.request_id

This will have to go in the tracing stub layer, because the tracing connection layer is never made aware of the request ID.