Fix W3CBaggagePropagator to allow empty baggage values per W3C spec (#8468)

Co-authored-by: Jay DeLuca <jaydeluca4@gmail.com>

Author: dahyvuun

api/all/src/main/java/io/opentelemetry/api/baggage/propagation/Element.java

@@ -31,6 +31,7 @@ class Element {
   }
 
   private final BitSet excluded;
+  private final boolean allowEmpty;
 
   private boolean leadingSpace;
   private boolean readingValue;
@@ -40,20 +41,21 @@ class Element {
   @Nullable private String value;
 
   static Element createKeyElement() {
-    return new Element(EXCLUDED_KEY_CHARS);
+    return new Element(EXCLUDED_KEY_CHARS, /* allowEmpty= */ false);
   }
 
   static Element createValueElement() {
-    return new Element(EXCLUDED_VALUE_CHARS);
+    return new Element(EXCLUDED_VALUE_CHARS, /* allowEmpty= */ true);
   }
 
   /**
    * Constructs element instance.
    *
    * @param excluded characters that are not allowed for this type of an element
    */
-  private Element(BitSet excluded) {
+  private Element(BitSet excluded, boolean allowEmpty) {
     this.excluded = excluded;
+    this.allowEmpty = allowEmpty;
     reset(0);
   }
 
@@ -77,8 +79,11 @@ boolean tryTerminating(int index, String header) {
     if (this.trailingSpace) {
       setValue(header);
       return true;
+    } else if (allowEmpty) {
+      // no content: empty value is valid (e.g. "key=" or "key=   "), empty key is not
+      this.value = "";
+      return true;
     } else {
-      // leading spaces - no content, invalid
       return false;
     }
   }

api/all/src/test/java/io/opentelemetry/api/baggage/propagation/W3CBaggagePropagatorTest.java

@@ -209,7 +209,8 @@ void extract_value_empty() {
     Context result =
         propagator.extract(Context.root(), ImmutableMap.of("baggage", "key1="), getter);
 
-    assertThat(Baggage.fromContext(result)).isEqualTo(Baggage.empty());
+    Baggage expectedBaggage = Baggage.builder().put("key1", "").build();
+    assertThat(Baggage.fromContext(result)).isEqualTo(expectedBaggage);
   }
 
   @Test
@@ -220,7 +221,9 @@ void extract_value_empty_withMetadata() {
         propagator.extract(
             Context.root(), ImmutableMap.of("baggage", "key1=;metakey=metaval"), getter);
 
-    assertThat(Baggage.fromContext(result)).isEqualTo(Baggage.empty());
+    Baggage expectedBaggage =
+        Baggage.builder().put("key1", "", BaggageEntryMetadata.create("metakey=metaval")).build();
+    assertThat(Baggage.fromContext(result)).isEqualTo(expectedBaggage);
   }
 
   @Test
@@ -230,7 +233,8 @@ void extract_value_onlySpaces() {
     Context result =
         propagator.extract(Context.root(), ImmutableMap.of("baggage", "key1=     "), getter);
 
-    assertThat(Baggage.fromContext(result)).isEqualTo(Baggage.empty());
+    Baggage expectedBaggage = Baggage.builder().put("key1", "").build();
+    assertThat(Baggage.fromContext(result)).isEqualTo(expectedBaggage);
   }
 
   @Test