fix(spanner): honor built-in metrics opt-out for gRPC metrics exporter (#12711)

Fixes: #12229

setBuiltInMetricsEnabled(false) was only disabling the Spanner API
tracer metrics path. During channel setup, the client could still
install gRPC built-in metrics/export wiring and create the Cloud
Monitoring exporter. In practice, users could
still see SpannerCloudMonitoringExporter attempts and
monitoring.timeSeries.create permission failures even though startup
logs reported:
```
  Built-in metrics enabled: false
```
That behavior contradicted the documented contract that
setBuiltInMetricsEnabled(false) opts out of client-side metrics.

Author: rahul2393

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

@@ -2446,7 +2446,7 @@ public OpenTelemetry getBuiltInOpenTelemetry() {
   }
 
   public void enablegRPCMetrics(InstantiatingGrpcChannelProvider.Builder channelProviderBuilder) {
-    if (SpannerOptions.environment.isEnableGRPCBuiltInMetrics()) {
+    if (isEnableBuiltInMetrics() && SpannerOptions.environment.isEnableGRPCBuiltInMetrics()) {
       this.builtInMetricsProvider.enableGrpcMetrics(
           channelProviderBuilder,
           this.getProjectId(),

java-spanner/google-cloud-spanner/src/test/java/com/google/cloud/spanner/spi/v1/GapicSpannerRpcTest.java

@@ -1030,6 +1030,34 @@ public void testGrpcGcpOtelMetricsDisabledSkipsMeterInjection() throws Exception
     assertNull(metricsOptions.getOpenTelemetryMeter());
   }
 
+  @Test
+  public void testBuiltInMetricsDisabledSkipsGrpcBuiltInMetricsConfigurator() {
+    try {
+      SpannerOptions.useEnvironment(
+          new SpannerOptions.SpannerEnvironment() {
+            @Override
+            public boolean isEnableGRPCBuiltInMetrics() {
+              return true;
+            }
+          });
+
+      SpannerOptions options =
+          SpannerOptions.newBuilder()
+              .setProjectId("[PROJECT]")
+              .setCredentials(STATIC_CREDENTIALS)
+              .setBuiltInMetricsEnabled(false)
+              .build();
+      InstantiatingGrpcChannelProvider.Builder channelProviderBuilder =
+          InstantiatingGrpcChannelProvider.newBuilder();
+
+      options.enablegRPCMetrics(channelProviderBuilder);
+
+      assertNull(channelProviderBuilder.getChannelConfigurator());
+    } finally {
+      SpannerOptions.useDefaultEnvironment();
+    }
+  }
+
   private static final class RecordingTransportChannelProvider implements TransportChannelProvider {
     private final String host;
     private final int port;