Empty attribute values in disk buffering (#2268)

Co-authored-by: otelbot <197425009+otelbot@users.noreply.github.com>

disk-buffering/src/main/java/io/opentelemetry/contrib/disk/buffering/internal/serialization/mapping/common/AttributesMapper.java

@@ -85,7 +85,13 @@ public final class AttributesMapper {
     } else if (value.array_value != null) {
       addArray(builder, key, value.array_value);
     } else {
-      throw new UnsupportedOperationException();
+      // Until we have complex attribute types that could potentially yield
+      // empty objects, we MUST assume here that the writer put an empty string
+      // into the value of the attribute. This will need to change later, when complex
+      // types arrive and the spec issue is resolved.
+      //
+      // See spec issue: https://github.com/open-telemetry/opentelemetry-specification/issues/4660
+      builder.put(AttributeKey.stringKey(key), "");
     }
   }
 

disk-buffering/src/test/java/io/opentelemetry/contrib/disk/buffering/internal/serialization/mapping/common/AttributesMapperTest.java

@@ -21,6 +21,7 @@ class AttributesMapperTest {
     Attributes attributes =
         Attributes.builder()
             .put(AttributeKey.stringKey("someString"), "someValue")
+            .put(AttributeKey.stringKey("emptyString"), "")
             .put(AttributeKey.booleanKey("someBool"), true)
             .put(AttributeKey.longKey("someLong"), 10L)
             .put(AttributeKey.doubleKey("someDouble"), 10.0)

disk-buffering/src/test/java/io/opentelemetry/contrib/disk/buffering/testutils/TestData.java

@@ -37,6 +37,8 @@ public final class TestData {
           .put("conditions", false, true)
           .put("scores", 0L, 1L)
           .put("coins", 0.01, 0.05, 0.1)
+          .put("empty", "")
+          .put("blank", " ")
           .build();
 
   public static final Resource RESOURCE_FULL =