Fix: preserve new baggage values in context on key conflict

micrometer-tracing/src/main/java/io/micrometer/tracing/contextpropagation/reactor/ReactorBaggage.java

@@ -59,10 +59,11 @@ public static Function<Context, Context> append(Map<String, String> baggage) {
     private static Context append(Context context, Map<String, String> baggage) {
         BaggageToPropagate baggageToPropagate = context.getOrDefault(ObservationAwareBaggageThreadLocalAccessor.KEY,
                 null);
-        Map<String, String> mergedBaggage = new HashMap<>(baggage);
+        Map<String, String> mergedBaggage = new HashMap<>();
         if (baggageToPropagate != null) {
             mergedBaggage.putAll(baggageToPropagate.getBaggage());
         }
+        mergedBaggage.putAll(baggage);
         BaggageToPropagate merged = new BaggageToPropagate(mergedBaggage);
         return context.put(ObservationAwareBaggageThreadLocalAccessor.KEY, merged);
     }
@@ -71,10 +72,10 @@ private static Context append(Context context, String key, String value) {
         BaggageToPropagate baggageToPropagate = context.getOrDefault(ObservationAwareBaggageThreadLocalAccessor.KEY,
                 null);
         Map<String, String> mergedBaggage = new HashMap<>();
-        mergedBaggage.put(key, value);
         if (baggageToPropagate != null) {
             mergedBaggage.putAll(baggageToPropagate.getBaggage());
         }
+        mergedBaggage.put(key, value);
         BaggageToPropagate merged = new BaggageToPropagate(mergedBaggage);
         return context.put(ObservationAwareBaggageThreadLocalAccessor.KEY, merged);
     }

micrometer-tracing/src/test/java/io/micrometer/tracing/contextpropagation/reactor/ReactorBaggageTests.java

@@ -0,0 +1,87 @@
+/**
+ * Copyright 2025 the original author or authors.
+ * <p>
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
+ * in compliance with the License. You may obtain a copy of the License at
+ * <p>
+ * https://www.apache.org/licenses/LICENSE-2.0
+ * <p>
+ * Unless required by applicable law or agreed to in writing, software distributed under the License
+ * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
+ * or implied. See the License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package io.micrometer.tracing.contextpropagation.reactor;
+
+import io.micrometer.tracing.contextpropagation.BaggageToPropagate;
+import io.micrometer.tracing.contextpropagation.ObservationAwareBaggageThreadLocalAccessor;
+import org.junit.jupiter.api.Test;
+import reactor.util.context.Context;
+
+import java.util.HashMap;
+import java.util.Map;
+
+import static org.assertj.core.api.BDDAssertions.then;
+
+class ReactorBaggageTests {
+
+    @Test
+    void should_append_single_baggage_entry_to_context() {
+        Context updatedContext = ReactorBaggage.append("foo", "bar").apply(Context.empty());
+
+        BaggageToPropagate baggage = updatedContext.get(ObservationAwareBaggageThreadLocalAccessor.KEY);
+        then(baggage).isNotNull();
+        then(baggage.getBaggage()).hasSize(1);
+        then(baggage.getBaggage()).containsEntry("foo", "bar");
+    }
+
+    @Test
+    void should_append_multiple_baggage_entries_to_context() {
+        Map<String, String> baggageEntries = new HashMap<>();
+        baggageEntries.put("foo", "bar");
+        baggageEntries.put("baz", "bar2");
+
+        Context updatedContext = ReactorBaggage.append(baggageEntries).apply(Context.empty());
+
+        BaggageToPropagate baggage = updatedContext.get(ObservationAwareBaggageThreadLocalAccessor.KEY);
+        then(baggage).isNotNull();
+        then(baggage.getBaggage()).hasSize(2);
+        then(baggage.getBaggage()).containsExactlyInAnyOrderEntriesOf(baggageEntries);
+    }
+
+    @Test
+    void should_merge_existing_baggage_with_new_entries() {
+        Map<String, String> initialBaggage = new HashMap<>();
+        initialBaggage.put("foo", "bar");
+        initialBaggage.put("baz", "bar2");
+        Context context = Context.of(ObservationAwareBaggageThreadLocalAccessor.KEY,
+                new BaggageToPropagate(initialBaggage));
+
+        Map<String, String> newBaggage = new HashMap<>();
+        newBaggage.put("foo", "bar1");
+
+        Context updatedContext = ReactorBaggage.append(newBaggage).apply(context);
+
+        BaggageToPropagate baggage = updatedContext.get(ObservationAwareBaggageThreadLocalAccessor.KEY);
+        then(baggage).isNotNull();
+        then(baggage.getBaggage()).hasSize(2);
+        then(baggage.getBaggage()).containsEntry("foo", "bar1").containsEntry("baz", "bar2");
+    }
+
+    @Test
+    void should_merge_existing_baggage_with_single_entry() {
+        Map<String, String> initialBaggage = new HashMap<>();
+        initialBaggage.put("foo", "bar");
+        initialBaggage.put("baz", "bar2");
+        Context context = Context.of(ObservationAwareBaggageThreadLocalAccessor.KEY,
+                new BaggageToPropagate(initialBaggage));
+
+        Context updatedContext = ReactorBaggage.append("foo", "bar1").apply(context);
+
+        BaggageToPropagate baggage = updatedContext.get(ObservationAwareBaggageThreadLocalAccessor.KEY);
+        then(baggage).isNotNull();
+        then(baggage.getBaggage()).hasSize(2);
+        then(baggage.getBaggage()).containsEntry("foo", "bar1").containsEntry("baz", "bar2");
+    }
+
+}