Internal: Add span events native serialization

[marcotc]

Add native serialization for the Datadog::Tracing::SpanEvent object, when serialized as MessagePack.

This code disabled today, through the native_events_supported variable, so this PR is just adding the serialization code that will be activated in a future PR.

Change log entry
None

Additional Notes:

I've also changed the returned keys from to_hash to have String keys, instead of Symbols from before, because both the MessagePack and JSON serialization will convert all this Symbols to Strings immediately. Making this change avoids an unnecessary Symbol -> String conversion.

How to test the change?
All changes are tested, as well as some improved testing for existing Span Events methods.

[datadog-datadog-prod-us1]

Datadog Report

Branch report: span-events-native
Commit report: f79fd65
Test service: dd-trace-rb

✅ 0 Failed, 22112 Passed, 1458 Skipped, 5m 47.08s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-12-11 20:21:27

Comparing candidate commit afce240 in PR branch span-events-native with baseline commit b985e32 in branch master.

Found 0 performance improvements and 1 performance regressions! Performance is the same for 30 metrics, 2 unstable metrics.

scenario:profiler - sample timeline=false

• 🟥 throughput [-0.349op/s; -0.341op/s] or [-7.248%; -7.076%]

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.07692% with 9 lines in your changes missing coverage. Please review.

Project coverage is 97.76%. Comparing base (b985e32) to head (afce240).
Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/datadog/tracing/span_event.rb	88.52%	7 Missing ⚠️
...tadog/tracing/transport/serializable_trace_spec.rb	92.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #4218    +/-   ##
========================================
  Coverage   97.75%   97.76%            
========================================
  Files        1357     1357            
  Lines       82130    82238   +108     
  Branches     4174     4205    +31     
========================================
+ Hits        80288    80398   +110     
+ Misses       1842     1840     -2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.