Description

According to Zipkin's documentation, the TraceID passed via HTTP headers uses lowercase hexadecimal encoding, not binary data. For example, the B3 specification states that the value of the X-B3-TraceId header is 16 or 32 lowercase hexadecimal characters (corresponding to 64 or 128 bits, respectively)

• https://zipkin.io/pages/instrumenting.html#:~:text=Ids%20are%20encoded%20as%20hex,strings
• https://github.com/openzipkin/b3-propagation?tab=readme-ov-file#traceid-1

We need to convert the trace_id and span_id formats stored in the variables.

Which issue(s) this PR fixes:

Fixes #12271

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)