Refactor span events

[svetlanabrennan]

Description

Please provide a brief description of the changes introduced in this pull request.
What problem does it solve? What is the context of this change?

How to Test

Please describe how you have tested these changes. Have you run the code against an example application?
What steps did you take to ensure that the changes are working correctly?

Related Issues

Please include any related issues or pull requests in this section, using the format Closes #<issue number> or Fixes #<issue number> if applicable.

[codecov]

Codecov Report

❌ Patch coverage is 71.22862% with 185 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.94%. Comparing base (b4f0010) to head (04e0fb5).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/spans/span.js	70.92%	164 Missing ⚠️
lib/spans/span-event.js	76.74%	10 Missing ⚠️
lib/agent.js	28.57%	5 Missing ⚠️
lib/spans/span-aggregator.js	50.00%	5 Missing ⚠️
lib/transaction/trace/partial-trace.js	75.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3628       +/-   ##
===========================================
+ Coverage   53.80%   80.94%   +27.13%     
===========================================
  Files         364      429       +65     
  Lines       50643    56292     +5649     
  Branches        1        1               
===========================================
+ Hits        27248    45565    +18317     
+ Misses      23395    10727    -12668     

Flag	Coverage Δ
integration-tests-esm-20.x	52.69% <37.18%> (?)
integration-tests-esm-24.x	?
versioned-tests-22.x	80.84% <71.22%> (?)

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