Serialize span events via a dedicated field

[marcotc]

What does this PR do?
Before this PR, tracing span events were serialized as a JSON string in the span tag events. This causes issues with size limit, as tags can only be 5000 characters. This limit is problematic when reporting multiple span events, or when reporting large data in a span events, like a stack trace.

This PR uses a new top-level span field, span_events, which supports the full span event representation, without sizing limitation or artificial stringification that was required in the JSON serialization from before.

Because users can already use span events through the events tag, and agent support for the top-level field is scheduled for a future agent version, this change check with the agent before sending span events it in the new format.

The serialization logic is already merged in #4218

Change log entry
Remove tracing Span Events serialization limitations.

How to test the change?
There are system-tests, as well as unit tests for this change.

[github-actions]

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^{Visited at: 2025-01-23 21:25:08 UTC}

[datadog-datadog-prod-us1]

Datadog Report

Branch report: span-events-writer
Commit report: 148f1b0
Test service: dd-trace-rb

✅ 0 Failed, 22228 Passed, 1477 Skipped, 7m 5.72s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-02-06 00:45:14

Comparing candidate commit b6c3435 in PR branch span-events-writer with baseline commit a444023 in branch master.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 31 metrics, 2 unstable metrics.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.74%. Comparing base (a444023) to head (b6c3435).

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #4279    +/-   ##
========================================
  Coverage   97.73%   97.74%            
========================================
  Files        1368     1369     +1     
  Lines       83014    83133   +119     
  Branches     4221     4227     +6     
========================================
+ Hits        81134    81256   +122     
+ Misses       1880     1877     -3     

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[p-datadog]

I read the diff but couldn't find where the new serialization is actually implemented, could you maybe merge/rebase on master?

[marcotc]

@p-datadog I implemented the serialization in a previous PR: #4218

I should have mentioned that in this PR description, let me update it!

[datadog-datadog-prod-us1]

Datadog Report

Branch report: span-events-writer
Commit report: b6c3435
Test service: dd-trace-rb

✅ 0 Failed, 22112 Passed, 1479 Skipped, 5m 36.15s Total Time

[marcotc]

The system-tests issues happens because of the way test "skips" and force-tests-list interact: DataDog/system-tests#3904

[TonyCTHsu]

Can we refactor the method signature for native_events_supported ?

Prefer keyword argument to positional argument.

[Strech]

Awesome PR, I have just 1 minor thing