Add UnmarshalJSON method for TraceID

[RitaGlushkova]

I had to manually pass TraceID from my Kafka Producer to Consumer for one of my projects and realised that the UnmarshalJSON method for TraceID wasn't there.

So I had to manually convert my TraceID into a string before sending it to Kafka.
So, adding this method to the source would be helpful to avoid this extra manual work.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: RitaGlushkova / name: Margarita Glushkova (bbeec46)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.7%. Comparing base (0446207) to head (a18edd6).
Report is 1333 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3682     +/-   ##
=======================================
- Coverage   79.7%   79.7%   -0.1%     
=======================================
  Files        171     171             
  Lines      12673   12684     +11     
=======================================
+ Hits       10105   10112      +7     
- Misses      2355    2359      +4     
  Partials     213     213             

Files with missing lines	Coverage Δ
trace/trace.go	98.3% <100.0%> (+0.1%)	⬆️

... and 1 file with indirect coverage changes

[dmathieu]

See related discussion: #1819

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 14 days.

[pellared]

Given SpanID and TraceID are simple value objects and they already contain MarshalJSON, I think it makes sense to provide UnmarshalJSON as well.

@open-telemetry/go-maintainers WDYT?

[dashpole]

No concerns from me

[dmathieu]

No concerns from me, but there were valid concerns raised in the issue mentioned above.