Injected X-B3-Traceid header may be less then 16 characters long

[sullerandras]

It seems like the injectTextMap function does not add leading zeroes to the traceid when it's emitted in hexadecimal:

dd-trace-go/ddtrace/tracer/textmap.go

Lines 309 to 310 in 9495abe

	writer.Set(b3TraceIDHeader, strconv.FormatUint(ctx.traceID, 16))
	writer.Set(b3SpanIDHeader, strconv.FormatUint(ctx.spanID, 16))

This causes issues in our Ruby app because that assumes the Traceid header will be at least 16 chars long:
https://github.com/twitter/finagle/blob/d2e7485782faeada129d43d7511167c31811df81/finagle-thrift/src/main/ruby/lib/finagle-thrift/trace.rb#L103

Would it be possible to format the traceid in the injected headers to be exactly 16 chars long? E.g. something like fmt.Sprintf("%016x", ctx.traceID)

[knusbaum]

@sullerandras
I don't think that adding this would be a problem.

It's not made crystal-clear, but I think the B3 specification says that these headers should be 16 (or possibly 32 for traceid) characters anyway, so I think we should change this.

Would you be willing to put up a PR?