format b3 headers as 16 chars when 64 bit

[castengo]

Since b3 trace IDs can be either 64 or 128 bit, we should check the size of the traceID and encode it according to its size instead of always encoding it as 32 chars.

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[castengo]

@googlebot I signed it!

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[castengo]

I can either do this or check if the integer is between -9,223,372,036,854,775,808 and 9,223,372,036,854,775,807 in a guard. Thought this might be better?

[castengo]

I can also do binary:encode_unsigned(TraceId) and check if its size 64 or 128 before formatting it. Let me know which approach you prefer.