fix(kafka): [Queue Instrumentation 21] Preserve third-party baggage on Kafka producer records

`SentryKafkaProducerInterceptor.injectHeaders(...)` previously removed and
overwrote the outgoing `baggage` header on every record, discarding any
third-party baggage entries already present (e.g. set by another vendor's
instrumentation or the application itself).

Read the existing `baggage` header values off the `ProducerRecord` and
pass them to `TracingUtils.trace(...)`. The downstream
`BaggageHeader.fromBaggageAndOutgoingHeader` preserves non-`sentry-*`
entries in the outgoing header while Sentry continues to own its own
keys.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: adinauer

sentry-kafka/src/main/java/io/sentry/kafka/SentryKafkaProducerInterceptor.java

@@ -13,10 +13,13 @@
 import io.sentry.util.SpanUtils;
 import io.sentry.util.TracingUtils;
 import java.nio.charset.StandardCharsets;
+import java.util.ArrayList;
+import java.util.List;
 import java.util.Map;
 import org.apache.kafka.clients.producer.ProducerInterceptor;
 import org.apache.kafka.clients.producer.ProducerRecord;
 import org.apache.kafka.clients.producer.RecordMetadata;
+import org.apache.kafka.common.header.Header;
 import org.apache.kafka.common.header.Headers;
 import org.jetbrains.annotations.ApiStatus;
 import org.jetbrains.annotations.NotNull;
@@ -97,8 +100,10 @@ public void close() {}
   public void configure(final @Nullable Map<String, ?> configs) {}
 
   private void injectHeaders(final @NotNull Headers headers, final @NotNull ISpan span) {
+    final @Nullable List<String> existingBaggageHeaders =
+        readHeaderValues(headers, BaggageHeader.BAGGAGE_HEADER);
     final @Nullable TracingUtils.TracingHeaders tracingHeaders =
-        TracingUtils.trace(scopes, null, span);
+        TracingUtils.trace(scopes, existingBaggageHeaders, span);
     if (tracingHeaders != null) {
       final @NotNull SentryTraceHeader sentryTraceHeader = tracingHeaders.getSentryTraceHeader();
       headers.remove(sentryTraceHeader.getName());
@@ -120,4 +125,19 @@ private void injectHeaders(final @NotNull Headers headers, final @NotNull ISpan
         String.valueOf(DateUtils.millisToSeconds(System.currentTimeMillis()))
             .getBytes(StandardCharsets.UTF_8));
   }
+
+  private static @Nullable List<String> readHeaderValues(
+      final @NotNull Headers headers, final @NotNull String name) {
+    @Nullable List<String> values = null;
+    for (final @NotNull Header header : headers.headers(name)) {
+      final byte @Nullable [] value = header.value();
+      if (value != null) {
+        if (values == null) {
+          values = new ArrayList<>();
+        }
+        values.add(new String(value, StandardCharsets.UTF_8));
+      }
+    }
+    return values;
+  }
 }

sentry-kafka/src/test/kotlin/io/sentry/kafka/SentryKafkaProducerInterceptorTest.kt

@@ -1,5 +1,6 @@
 package io.sentry.kafka
 
+import io.sentry.BaggageHeader
 import io.sentry.IScopes
 import io.sentry.ISentryLifecycleToken
 import io.sentry.Sentry
@@ -79,6 +80,37 @@ class SentryKafkaProducerInterceptorTest {
     assertTrue(enqueuedTime > 0)
   }
 
+  @Test
+  fun `preserves pre-existing third-party baggage header entries`() {
+    val tx = createTransaction()
+    val interceptor = SentryKafkaProducerInterceptor<String, String>(scopes)
+    val record = ProducerRecord<String, String>("my-topic", "key", "value")
+    record
+      .headers()
+      .add(
+        BaggageHeader.BAGGAGE_HEADER,
+        "othervendor=someValue,another=thing".toByteArray(StandardCharsets.UTF_8),
+      )
+
+    interceptor.onSend(record)
+
+    val baggageHeaders = record.headers().headers(BaggageHeader.BAGGAGE_HEADER).toList()
+    assertEquals(1, baggageHeaders.size)
+    val baggageValue = String(baggageHeaders.first().value(), StandardCharsets.UTF_8)
+    assertTrue(
+      baggageValue.contains("othervendor=someValue"),
+      "expected third-party baggage entry preserved, got: $baggageValue",
+    )
+    assertTrue(
+      baggageValue.contains("another=thing"),
+      "expected third-party baggage entry preserved, got: $baggageValue",
+    )
+    assertTrue(
+      baggageValue.contains("sentry-"),
+      "expected Sentry baggage entries appended, got: $baggageValue",
+    )
+  }
+
   @Test
   fun `does not create span when queue tracing is disabled`() {
     val tx = createTransaction()