Update javadoc to allow null/empty attr values (#5616)

api/all/src/main/java/io/opentelemetry/api/trace/Span.java

@@ -91,8 +91,7 @@ public interface Span extends ImplicitContextKeyed {
    * Sets an attribute to the {@code Span}. If the {@code Span} previously contained a mapping for
    * the key, the old value is replaced by the specified value.
    *
-   * <p>If a null or empty String {@code value} is passed in, the behavior is undefined, and hence
-   * strongly discouraged.
+   * <p>Empty String "" and null are valid attribute {@code value}, but not valid keys.
    *
    * <p>Note: It is strongly recommended to use {@link #setAttribute(AttributeKey, Object)}, and
    * pre-allocate your keys, if possible.

api/all/src/test/java/io/opentelemetry/api/common/AttributesTest.java

@@ -578,4 +578,11 @@ class AttributesTest {
     assertThatCode(() -> myAttributesBuilder.remove(stringKey("foo"))).doesNotThrowAnyException();
     assertThatCode(() -> myAttributesBuilder.removeIf(unused -> false)).doesNotThrowAnyException();
   }
+
+  @Test
+  void emptyValueIsValid() {
+    AttributeKey<String> key = stringKey("anything");
+    Attributes attributes = Attributes.of(key, "");
+    assertThat(attributes.get(key)).isEqualTo("");
+  }
 }