View traceid for buildx invocation

[felixdesouza]

I've been playing around with the OpenTelemetry w/buildx which works great when using the browser.

However, I would like to be able to call docker buildx build ... and then see the traceid at the end. I can't figure out how to correlate individual runs against their traces. My best effort is generating some unique build arg, and searching through traces until I see it in the spans, but that feels unergonomic. Or if it's in the metadata file. I'm not really fussed where as long as I can get it following an invocation.