Deprecate TextMapGetter keys method

context/src/main/java/io/opentelemetry/context/propagation/TextMapGetter.java

@@ -16,15 +16,23 @@
  *
  * @param <C> carrier of propagation fields, such as an http request.
  */
+@FunctionalInterface
 public interface TextMapGetter<C> {
 
   /**
    * Returns all the keys in the given carrier.
    *
+   * <p>The default method returns an empty {@link Iterable}.
+   *
    * @param carrier carrier of propagation fields, such as an http request.
    * @since 0.10.0
+   * @deprecated Propagators should use {@link #get(Object, String)} or {@link #getAll(Object,
+   *     String)} with known fields instead.
    */
-  Iterable<String> keys(C carrier);
+  @Deprecated
+  default Iterable<String> keys(C carrier) {
+    return Collections.emptyList();
+  }
 
   /**
    * Returns the first value of the given propagation {@code key} or returns {@code null}.

context/src/test/java/io/opentelemetry/context/propagation/TextMapGetterTest.java

@@ -45,6 +45,15 @@ public String get(@Nullable Void carrier, String key) {
         }
       };
 
+  @Test
+  @SuppressWarnings("deprecation") // Verifies the default behavior of deprecated keys().
+  void canBeImplementedAsLambda() {
+    TextMapGetter<Void> getter = (carrier, key) -> "value";
+
+    assertThat(getter.get(null, "key")).isEqualTo("value");
+    assertThat(getter.keys(null)).isEqualTo(Collections.emptyList());
+  }
+
   @Test
   void extendedTextMapGetterdefaultMethod_returnsEmpty() {
     Iterator<String> result = nullGet.getAll(null, "key");

docs/apidiffs/current_vs_latest/opentelemetry-context.txt

@@ -1,2 +1,7 @@
 Comparing source compatibility of opentelemetry-context-1.64.0-SNAPSHOT.jar against opentelemetry-context-1.63.0.jar
-No changes.
+***  MODIFIED INTERFACE: PUBLIC ABSTRACT io.opentelemetry.context.propagation.TextMapGetter  (not serializable)
+	===  CLASS FILE FORMAT VERSION: 52.0 <- 52.0
+	GENERIC TEMPLATES: === C:java.lang.Object
+	***  MODIFIED METHOD: PUBLIC NON_ABSTRACT (<- ABSTRACT) java.lang.Iterable<java.lang.String><java.lang.String> keys(java.lang.Object)
+		+++  NEW ANNOTATION: java.lang.Deprecated
+	+++  NEW ANNOTATION: java.lang.FunctionalInterface

extensions/trace-propagators/src/main/java/io/opentelemetry/extension/trace/propagation/JaegerPropagator.java

@@ -244,6 +244,7 @@ private static <C> SpanContext getSpanContextFromHeader(
   }
 
   @Nullable
+  @SuppressWarnings("deprecation") // Jaeger baggage extraction requires enumerating carrier keys.
   private static <C> Baggage getBaggageFromHeader(@Nullable C carrier, TextMapGetter<C> getter) {
     BaggageBuilder builder = null;
     int entriesAdded = 0;

extensions/trace-propagators/src/main/java/io/opentelemetry/extension/trace/propagation/OtTracePropagator.java

@@ -109,6 +109,7 @@ public <C> void inject(Context context, @Nullable C carrier, TextMapSetter<C> se
   }
 
   @Override
+  @SuppressWarnings("deprecation") // OT trace baggage extraction requires enumerating carrier keys.
   public <C> Context extract(Context context, @Nullable C carrier, TextMapGetter<C> getter) {
     if (context == null) {
       return Context.root();