test(o11y): add static factory creation tests for metrics recorder

Added unit tests to `GoldenSignalsMetricsRecorderTest` to explicitly verify that `GoldenSignalsMetricsRecorder.create()` correctly intercepts invalid `LibraryMetadata` states (null values, missing artifact name, empty artifact name) and returns `null` as expected.

Author: blakeli0

sdk-platform-java/gax-java/gax/src/test/java/com/google/api/gax/tracing/GoldenSignalsMetricsRecorderTest.java

@@ -120,4 +120,29 @@ void recordOperationLatency_shouldRecordMetricAttributes() {
     assertThat(metricData.getHistogramData().getPoints().iterator().next().getAttributes())
         .isEqualTo(Attributes.of(AttributeKey.stringKey(ATTRIBUTE_1), VALUE_1));
   }
+
+  @Test
+  void create_shouldReturnNull_whenLibraryMetadataIsNull() {
+    GoldenSignalsMetricsRecorder actual =
+        GoldenSignalsMetricsRecorder.create(OpenTelemetry.noop(), null);
+    assertThat(actual).isNull();
+  }
+
+  @Test
+  void create_shouldReturnNull_whenArtifactNameIsNull() {
+    com.google.api.gax.rpc.LibraryMetadata metadata =
+        com.google.api.gax.rpc.LibraryMetadata.newBuilder().setVersion("1.0.0").build();
+    GoldenSignalsMetricsRecorder actual =
+        GoldenSignalsMetricsRecorder.create(OpenTelemetry.noop(), metadata);
+    assertThat(actual).isNull();
+  }
+
+  @Test
+  void create_shouldReturnNull_whenArtifactNameIsEmpty() {
+    com.google.api.gax.rpc.LibraryMetadata metadata =
+        com.google.api.gax.rpc.LibraryMetadata.newBuilder().setArtifactName("").build();
+    GoldenSignalsMetricsRecorder actual =
+        GoldenSignalsMetricsRecorder.create(OpenTelemetry.noop(), metadata);
+    assertThat(actual).isNull();
+  }
 }