Don't allocate for parentContext on span start for the common case

[jack-berg]

#8263 fixed a bug where parentContext wasn't properly being passed to shouldSample for the root case. Its correct, but the code calls parentContext.with(RANDOM_TRACE_ID_PRIMORDIAL_SPAN), which allocates to store the RANDOM_TRACE_ID_PRIMORDIAL_SPAN in context every time a root span is started.

But its common (I would say the majority of the time) for parent context to be Context.root() in this case. Therefore, we can avoid allocating by keeping a constant Context.root().with(RANDOM_TRACE_ID_PRIMORDIAL_SPAN) and always returning that when parentContext == Context.root().

Followup to #8263 (comment)

cc @PeterF778

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.25%. Comparing base (fdb8194) to head (7afdc6e).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8332      +/-   ##
============================================
- Coverage     90.27%   90.25%   -0.03%     
  Complexity     7693     7693              
============================================
  Files           850      850              
  Lines         23207    23210       +3     
  Branches       2356     2357       +1     
============================================
- Hits          20951    20948       -3     
- Misses         1530     1537       +7     
+ Partials        726      725       -1     

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