chore: disable native metrics when there are no credentials

Native metrics were automatically disabled when the emulator is used, but
some clients (e.g. PGAdapter) set up the connection to the emulator manually
instead of setting the environment variable. Also, when using in-mem mock
servers, native metrics should be disabled, as they cannot be exported.

This PR therefore adds an additional check that disables native metrics
when the client uses a NoCredentials instance.

google-cloud-spanner/src/main/java/com/google/cloud/spanner/SpannerOptions.java

@@ -83,6 +83,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Map.Entry;
+import java.util.Objects;
 import java.util.Set;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.ScheduledThreadPoolExecutor;
@@ -1791,6 +1792,10 @@ public CallCredentialsProvider getCallCredentialsProvider() {
     return callCredentialsProvider;
   }
 
+  public boolean usesNoCredentials() {
+    return Objects.equals(getCredentials(), NoCredentials.getInstance());
+  }
+
   public String getCompressorName() {
     return compressorName;
   }
@@ -1838,7 +1843,7 @@ private ApiTracerFactory createApiTracerFactory(
 
     // Add Metrics Tracer factory if built in metrics are enabled and if the client is data client
     // and if emulator is not enabled.
-    if (isEnableBuiltInMetrics() && !isAdminClient && !isEmulatorEnabled) {
+    if (isEnableBuiltInMetrics() && !isAdminClient && !isEmulatorEnabled && !usesNoCredentials()) {
       ApiTracerFactory metricsTracerFactory = createMetricsApiTracerFactory();
       if (metricsTracerFactory != null) {
         apiTracerFactories.add(metricsTracerFactory);

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

@@ -738,7 +738,7 @@ public void testLeaderAwareRoutingEnablement() {
 
   @Test
   public void testEndToEndTracingEnablement() {
-    // Test that end to end tracing is disabled by default.
+    // Test that end-to-end tracing is disabled by default.
     assertFalse(SpannerOptions.newBuilder().setProjectId("p").build().isEndToEndTracingEnabled());
     assertTrue(
         SpannerOptions.newBuilder()
@@ -755,7 +755,7 @@ public void testEndToEndTracingEnablement() {
   }
 
   @Test
-  public void testmonitoringHost() {
+  public void testMonitoringHost() {
     String metricsEndpoint = "test-endpoint:443";
     assertNull(SpannerOptions.newBuilder().setProjectId("p").build().getMonitoringHost());
     assertThat(