What Does This Do

This PR addresses my feedback from #11855:

• a single event collection per span instead of one in OtelSpan and one in DDSpan;
• no v0 encoding logic in the OTel classes;
• no core implementation details in internal-api.

About implementation details:

• Stores span events as DDSpanEvent objects on DDSpan, created in core through two new AgentSpan methods: addEvent(name, attributes) and addEvent(name, attributes, timestamp, unit). Previously the OTel shim JSON-encoded them into the events tag when the span finished. internal-api no longer has an event type.
• Encodes the events tag in core (DDSpanContext) at serialization time, next to the span links tag. This covers the tag-based protocols and writers: v0.4/v0.5, OTLP, CI Visibility, LLM Obs and file-based.
• Events are appended to a list under the span lock, so each add is cheap even for spans with many events. The list is frozen when the span finishes: later events are ignored, as the OTel shim already did after end(), and serialization uses the list without copying it. A copy is only taken when a span is serialized while still running, such as long-running spans.
• Writes structured events directly in v1 payloads (span field 12) so TraceMapperV1 no longer parses JSON.
• The v0.x tag keeps the previous StringBuilder encoding and escaping rules. It now uses one builder per tag instead of one per event and per attribute set. I didn't use datadog.json.JsonWriter: on JDK 17 and earlier its OutputStreamWriter allocates an 8 KB buffer per call, and it encodes one character at a time, which is costly for exception stack traces.
• Events without an explicit time are now timestamped with the tracer clock, the same one used for span start and end. The OTel shim previously used a static SystemTimeSource, which has only millisecond precision.
• All JSON encoding code is removed from the OTel shim.

Motivation

Events were JSON-encoded on the application thread for every protocol. v1 then parsed that JSON back to write native events: encode → decode → re-encode.
This also moves work off the application thread: attributes are copied into a map instead of being JSON-encoded when an event is added, and no tag is built when the span finishes.

Additional Notes

Intended behaviour changes:

• The events tag is only added when the span is serialized, so trace interceptors no longer see it. This already applies to _dd.span_links.

Fixes:

• Event names are now escaped in the events tag. A name containing ", \ or a control character used to produce invalid JSON, or to be read as a different name. Names without those characters are encoded exactly as before.
• Span events recorded without a timestamp could get a time up to 1 ms before the span start, because they didn't use the span's clock. They now use the tracer clock.

Existing known limitations, to address separately:

• There is still no limit on the number of events per span, as before. The OTel SDK caps them at 128 by default.

Contributor Checklist

• Format the title according to the contribution guidelines
• Assign the type: and (comp: or inst:) labels in addition to any other useful labels
• Avoid using close, fix, or any linking keywords when referencing an issue
  Use solves instead, and assign the PR milestone to the issue
• Update the CODEOWNERS file on source file addition, migration, or deletion
• Update public documentation with any new configuration flags or behaviors
• Once approved, use merge queue to merge the PR

Jira ticket: [PROJ-IDENT]

🤖 Generated with Claude Code