test(spanner): fix built-in metrics emulator unit test (#14255)

Author: rahul2393

java-spanner/google-cloud-spanner/src/test/java/com/google/cloud/spanner/BuiltInOpenTelemetryMetricsProviderTest.java

@@ -111,6 +111,10 @@ private void verifyHash(String hash) {
   private SpannerOptions newTestOptions() {
     return SpannerOptions.newBuilder()
         .setProjectId("host-project")
+        // The builder picks up SPANNER_EMULATOR_HOST from the environment, and building with an
+        // emulator host replaces the credentials below with NoCredentials. Built-in metrics are
+        // disabled for a client without credentials, so leave the emulator out of these tests.
+        .setEmulatorHost(null)
         .setCredentials(
             OAuth2Credentials.create(new AccessToken("test-token", new Date(Long.MAX_VALUE))))
         .build();