support validation errors for CI spans

[anmarchenko]

Motivation
Currently we don't see any feedback when invalid events are being discarded during serialization phase. This behaviour was inherited from ddtrace-rb where such errors are only emitted in debug mode.

For CI visibility library it is very important to report all the spans, so skipping invalid spans is a serious error that requires an action.

What does this PR do?

• Elevates logging level for invalid spans skipping to WARN
• Now event serializer stores validation errors so that they could be logged for easier debugging

How to test the change?
Unit tests are provided

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c42591d) 99.34% compared to head (b5938c5) 99.34%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #78   +/-   ##
=======================================
  Coverage   99.34%   99.34%           
=======================================
  Files         131      131           
  Lines        4754     4762    +8     
  Branches      166      166           
=======================================
+ Hits         4723     4731    +8     
  Misses         31       31           

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