Fix BaggageBuilder.put() silently accepting empty string keys

api/all/src/main/java/io/opentelemetry/api/baggage/BaggageBuilder.java

@@ -15,6 +15,10 @@ public interface BaggageBuilder {
   /**
    * Adds the key/value pair and metadata regardless of whether the key is present.
    *
+   * <p>Per the <a href="https://www.w3.org/TR/baggage/#definition">W3C Baggage spec</a>, a
+   * baggage-name must be a non-empty token. Calls with a {@code null} or empty {@code key}, a
+   * {@code null} {@code value}, or a {@code null} {@code entryMetadata} are silently ignored.
+   *
    * @param key the {@code String} key which will be set.
    * @param value the {@code String} value to set for the given key.
    * @param entryMetadata the {@code BaggageEntryMetadata} metadata to set for the given key.

api/all/src/main/java/io/opentelemetry/api/baggage/ImmutableBaggage.java

@@ -69,7 +69,7 @@ static class Builder implements BaggageBuilder {
 
     @Override
     public BaggageBuilder put(String key, String value, BaggageEntryMetadata entryMetadata) {
-      if ((key == null) || (value == null) || (entryMetadata == null)) {
+      if ((key == null) || key.isEmpty() || (value == null) || (entryMetadata == null)) {
         return this;
       }
       data.add(key);

api/all/src/test/java/io/opentelemetry/api/baggage/ImmutableBaggageTest.java

@@ -114,8 +114,17 @@ void put_keyUnprintableChars() {
   @Test
   void put_keyEmpty() {
     BaggageBuilder builder = ONE_ENTRY.toBuilder();
+    Baggage built = builder.build();
     builder.put("", "value");
-    assertThat(builder.build().getEntryValue("")).isEqualTo("value");
+    assertThat(builder.build()).isEqualTo(built);
+  }
+
+  @Test
+  void put_keyEmpty_withMetadata() {
+    BaggageBuilder builder = ONE_ENTRY.toBuilder();
+    Baggage built = builder.build();
+    builder.put("", "value", TMD);
+    assertThat(builder.build()).isEqualTo(built);
   }
 
   @Test