Lazy init of GUIDs on DefaultTracers

[jtduffy]

Closes #2057 from @yuzawa-san

The GUID for a tracer is only necessary if it's going to be reported up to NR. This change defers the creation of the GUID until the getGuid() call is made.

Another minor optimization was moving the hexchars const to a class level member.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.59%. Comparing base (857433f) to head (6fba5c8).
Report is 34 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #2088   +/-   ##
=========================================
  Coverage     70.59%   70.59%           
+ Complexity     9883     9881    -2     
=========================================
  Files           829      829           
  Lines         39961    39961           
  Branches       6080     6081    +1     
=========================================
  Hits          28211    28211           
- Misses         9020     9021    +1     
+ Partials       2730     2729    -1     

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