ActivitySpanId
A SpanId is the format the W3C standard requires for its ID for a single span in a trace.
It represents 8 binary bytes of information, typically displayed as 16 characters
of Hexadecimal. A SpanId is a STRUCT, and does contain the 8 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 SpanId with at random number in it (very likely to be unique)
Copy the bytes of the TraceId (8 bytes total) into the 'destination' span.
Returns the SpanId as a 16 character hexadecimal string.