Fix refactoring that broke reflection.

- fix reflective method lookup
- also add unit test in the right project.

Author: jsuereth

sdk/common/src/main/java/io/opentelemetry/sdk/resources/internal/EntityUtil.java

@@ -37,7 +37,7 @@ private EntityUtil() {}
   /** Appends a new entity on to the end of the list of entities. */
   public static final ResourceBuilder addEntity(ResourceBuilder rb, Entity e) {
     try {
-      Method method = Resource.class.getDeclaredMethod("add", Entity.class);
+      Method method = ResourceBuilder.class.getDeclaredMethod("add", Entity.class);
       if (method != null) {
         method.setAccessible(true);
         method.invoke(rb, e);
@@ -55,7 +55,7 @@ public static final ResourceBuilder addEntity(ResourceBuilder rb, Entity e) {
   /** Appends a new collection of entities on to the end of the list of entities. */
   public static final ResourceBuilder addAllEntity(ResourceBuilder rb, Collection<Entity> e) {
     try {
-      Method method = Resource.class.getDeclaredMethod("addAll", Collection.class);
+      Method method = ResourceBuilder.class.getDeclaredMethod("addAll", Collection.class);
       if (method != null) {
         method.setAccessible(true);
         method.invoke(rb, e);

sdk/common/src/test/java/io/opentelemetry/sdk/resources/internal/EntityUtilTest.java

@@ -9,6 +9,7 @@
 import static org.assertj.core.api.Assertions.assertThat;
 
 import io.opentelemetry.api.common.Attributes;
+import io.opentelemetry.sdk.resources.Resource;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
@@ -246,4 +247,28 @@ void testRawAttributeMerge_entity_with_conflict() {
         .containsEntry("b", 2)
         .containsEntry("c", 2);
   }
+
+  @Test
+  void testAddEntity_reflection() {
+    Resource result =
+        EntityUtil.addEntity(
+                Resource.builder(), Entity.builder("a").withId(id -> id.put("a", 1)).build())
+            .build();
+    assertThat(EntityUtil.getEntities(result))
+        .satisfiesExactlyInAnyOrder(e -> assertThat(e).hasType("a"));
+  }
+
+  @Test
+  void testAddAllEntity_reflection() {
+    Resource result =
+        EntityUtil.addAllEntity(
+                Resource.builder(),
+                Arrays.asList(
+                    Entity.builder("a").withId(id -> id.put("a", 1)).build(),
+                    Entity.builder("b").withId(id -> id.put("b", 1)).build()))
+            .build();
+    assertThat(EntityUtil.getEntities(result))
+        .satisfiesExactlyInAnyOrder(
+            e -> assertThat(e).hasType("a"), e -> assertThat(e).hasType("b"));
+  }
 }