fix(native-spans): keep OTLP span events on the native event slot

The `meta.events` JSON fallback exists for agents that cannot read the
native `span_events` field, gated on `DD_TRACE_NATIVE_SPAN_EVENTS`. That
gate is about the agent protocol, so with `OTEL_TRACES_EXPORTER=otlp` and
the default flag value every event reached the collector as a JSON string
attribute instead of a structured OTLP event, breaking consumers of
exception data. The deleted OTLP transformer converted events regardless
of the flag.

Take the native path whenever the destination is OTLP.

Reported by Codex review as P2.

Author: bengl

packages/dd-trace/src/native/span.js

@@ -559,15 +559,18 @@ class NativeDatadogSpan extends DatadogSpan {
    * shape the legacy JS encoder writes (`meta.events` via stringifySpanEvents),
    * which is what the agent expects when it doesn't support native span events
    * (system-tests Test_SpanEvents_WithoutAgentSupport).
+   *
+   * The meta fallback exists purely for agents that cannot read the native slot,
+   * so it must not apply to OTLP: libdatadog maps the native `span_events` into
+   * real OTLP events, whereas the meta tag would reach the collector as a JSON
+   * string attribute. The deleted OTLP transformer converted events regardless of
+   * this agent-protocol flag, so OTLP always takes the native path.
    */
   #serializeSpanEvents () {
     if (!this._events?.length) return
 
-    // When native span events are enabled (matching the legacy encoder's
-    // `DD_TRACE_NATIVE_SPAN_EVENTS` gate), append each event to the top-level
-    // v0.4 `span_events` field via the native setter — no truncation, typed
-    // attributes. Otherwise fall back to the `events` meta tag (plain JSON).
-    if (this.tracer()._config.DD_TRACE_NATIVE_SPAN_EVENTS) {
+    const config = this.tracer()._config
+    if (config.DD_TRACE_NATIVE_SPAN_EVENTS || config.OTEL_TRACES_EXPORTER === 'otlp') {
       for (const event of this._events) {
         // `addEvent` and the OTel bridge do not type-check `name`. A non-string
         // reaches the WASM string parameter and throws out of `finish()` into

packages/dd-trace/test/native/span.spec.js

@@ -678,6 +678,21 @@ describe('NativeDatadogSpan', () => {
       assert.strictEqual(nativeSpans.addSpanEvent.getCall(0).args[1], 'good')
     })
 
+    it('uses the native event slot for OTLP even when the agent flag is disabled', () => {
+      // The meta fallback exists for agents that cannot read the native slot. An
+      // OTLP collector would receive it as a JSON string attribute instead of
+      // structured events, so OTLP must always take the native path.
+      tracer._config.DD_TRACE_NATIVE_SPAN_EVENTS = false
+      tracer._config.OTEL_TRACES_EXPORTER = 'otlp'
+      span._events.push({ name: 'exception', startTime: 4 })
+
+      span.finish()
+
+      sinon.assert.calledOnce(nativeSpans.addSpanEvent)
+      assert.strictEqual(nativeSpans.addSpanEvent.getCall(0).args[1], 'exception')
+      assert.strictEqual(span._spanContext.getTag('events'), undefined)
+    })
+
     it('falls back to the `events` meta tag when the flag is disabled', () => {
       tracer._config.DD_TRACE_NATIVE_SPAN_EVENTS = false
       span._events.push({ name: 'evt', startTime: 1, attributes: { k: 'v' } })