Attribute assertions should always contain the attr key

sdk/testing/src/main/java/io/opentelemetry/sdk/testing/assertj/AttributeAssertion.java

@@ -37,6 +37,13 @@ static AttributeAssertion create(
   // suppressing here.
   @SuppressWarnings("NullAway")
   static AbstractAssert<?, ?> attributeValueAssertion(AttributeKey<?> key, @Nullable Object value) {
+    AbstractAssert<? extends AbstractAssert<?, ?>, ?> abstractAssert = makeAssertion(key, value);
+    String description = "%s attribute '%s'";
+    return abstractAssert.as(description, key.getType(), key.getKey());
+  }
+
+  private static AbstractAssert<? extends AbstractAssert<?, ?>, ?> makeAssertion(
+      AttributeKey<?> key, Object value) {
     switch (key.getType()) {
       case STRING:
         return assertThat((String) value);

sdk/testing/src/test/java/io/opentelemetry/sdk/testing/assertj/AttributeAssertionTest.java

@@ -0,0 +1,27 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.sdk.testing.assertj;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.junit.Assert.assertThrows;
+
+import io.opentelemetry.api.common.AttributeKey;
+import java.util.function.Consumer;
+import org.assertj.core.api.AbstractAssert;
+import org.junit.jupiter.api.Test;
+
+class AttributeAssertionTest {
+
+  @Test
+  void nullAttr_errorMessageContainsAttrName() {
+    AttributeKey<String> key = AttributeKey.stringKey("flib");
+    AttributeAssertion attrAssertion = AttributeAssertion.create(key, AbstractAssert::isNotNull);
+    AbstractAssert<?, ?> anAssert = AttributeAssertion.attributeValueAssertion(key, null);
+    Consumer<AbstractAssert<?, ?>> a = attrAssertion.getAssertion();
+    String message = assertThrows(AssertionError.class, () -> a.accept(anAssert)).getMessage();
+    assertThat(message).isEqualTo("[STRING attribute 'flib'] \nExpecting actual not to be null");
+  }
+}