Fix SpanEventSerializerTest flakiness

[ambushwork]

What does this PR do?

Forgery will generate attribute key like "id" sometimes, when the attribute is serialised, the key like this will be excluded because it's already declared in the main json structure, which causes test failure.

So in unit test we should exclude them to avoid flakiness

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.33%. Comparing base (4cc5caf) to head (3d85ee1).
Report is 4 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2311      +/-   ##
===========================================
+ Coverage    70.31%   70.33%   +0.01%     
===========================================
  Files          736      736              
  Lines        27456    27466      +10     
  Branches      4606     4607       +1     
===========================================
+ Hits         19305    19316      +11     
+ Misses        6862     6860       -2     
- Partials      1289     1290       +1     

see 35 files with indirect coverage changes