ActivityTraceId
A TraceId is the format the W3C standard requires for its ID for the entire trace.
It represents 16 binary bytes of information, typically displayed as 32 characters
of Hexadecimal. A TraceId is a STRUCT, and does contain the 16 bytes of binary information
so there is value in passing it by reference. It does know how to convert to and
from its Hexadecimal string representation, tries to avoid changing formats until
it has to, and caches the string representation after it was created.
It is mostly useful as an exchange type.
Create a new TraceId with at random number in it (very likely to be unique)
Copy the bytes of the TraceId (16 total) into the 'destination' span.
Returns the TraceId as a 32 character hexadecimal string.