feat(spanner): add setMetricsProjectId to fix metrics export

SpannerOptions.getProjectId() can resolve to the wrong project (e.g.
host project on GKE), causing createServiceTimeSeries to fail with
permission errors. Add resolveMetricsProjectId() which defaults to the
database project and setMetricsProjectId(String) to allow explicit
override when needed.

Author: RRap0so

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

@@ -306,6 +306,7 @@ static GcpChannelPoolOptions mergeWithDefaultChannelPoolOptions(
   private final boolean enableExtendedTracing;
   private final boolean enableEndToEndTracing;
   private final String monitoringHost;
+  private final String metricsProjectId;
   private final TransactionOptions defaultTransactionOptions;
   private final RequestOptions.ClientContext clientContext;
 
@@ -991,6 +992,7 @@ protected SpannerOptions(Builder builder) {
     }
     enableEndToEndTracing = builder.enableEndToEndTracing;
     monitoringHost = builder.monitoringHost;
+    metricsProjectId = builder.metricsProjectId;
     defaultTransactionOptions = builder.defaultTransactionOptions;
     clientContext = builder.clientContext;
   }
@@ -1251,6 +1253,7 @@ public static class Builder
     private boolean enableBuiltInMetrics = SpannerOptions.environment.isEnableBuiltInMetrics();
     private boolean enableLocationApi = SpannerOptions.environment.isEnableLocationApi();
     private String monitoringHost = SpannerOptions.environment.getMonitoringHost();
+    private String metricsProjectId;
     private SslContext mTLSContext = null;
     private String experimentalHost = null;
     private boolean usePlainText = false;
@@ -1360,6 +1363,7 @@ protected Builder() {
       this.enableLocationApi = options.enableLocationApi;
       this.enableEndToEndTracing = options.enableEndToEndTracing;
       this.monitoringHost = options.monitoringHost;
+      this.metricsProjectId = options.metricsProjectId;
       this.defaultTransactionOptions = options.defaultTransactionOptions;
       this.clientContext = options.clientContext;
     }
@@ -2033,6 +2037,17 @@ public Builder setMonitoringHost(String monitoringHost) {
       return this;
     }
 
+    /**
+     * Sets the GCP project ID for exporting client-side built-in metrics. Defaults to {@link
+     * SpannerOptions#getProjectId()} when not set. On GKE with shared VPC, the default project may
+     * resolve to the host project instead of the application project; set this explicitly to avoid
+     * {@code monitoring.metricWriter} permission errors.
+     */
+    public Builder setMetricsProjectId(String metricsProjectId) {
+      this.metricsProjectId = metricsProjectId;
+      return this;
+    }
+
     /**
      * Sets whether to enable extended OpenTelemetry tracing. Enabling this option will add the
      * following additional attributes to the traces that are generated by the client:
@@ -2443,14 +2458,14 @@ public ApiTracerFactory getApiTracerFactory() {
   @InternalApi
   public OpenTelemetry getBuiltInOpenTelemetry() {
     return this.builtInMetricsProvider.getOrCreateOpenTelemetry(
-        this.getProjectId(), getCredentials(), this.monitoringHost, getUniverseDomain());
+        this.resolveMetricsProjectId(), getCredentials(), this.monitoringHost, getUniverseDomain());
   }
 
   public void enablegRPCMetrics(InstantiatingGrpcChannelProvider.Builder channelProviderBuilder) {
     if (isEnableBuiltInMetrics() && SpannerOptions.environment.isEnableGRPCBuiltInMetrics()) {
       this.builtInMetricsProvider.enableGrpcMetrics(
           channelProviderBuilder,
-          this.getProjectId(),
+          this.resolveMetricsProjectId(),
           getCredentials(),
           this.monitoringHost,
           getUniverseDomain());
@@ -2499,7 +2514,7 @@ private ApiTracerFactory getDefaultApiTracerFactory() {
   private ApiTracerFactory createMetricsApiTracerFactory() {
     OpenTelemetry openTelemetry =
         this.builtInMetricsProvider.getOrCreateOpenTelemetry(
-            this.getProjectId(), getCredentials(), this.monitoringHost, getUniverseDomain());
+            this.resolveMetricsProjectId(), getCredentials(), this.monitoringHost, getUniverseDomain());
 
     return openTelemetry != null
         ? new BuiltInMetricsTracerFactory(
@@ -2543,6 +2558,14 @@ String getMonitoringHost() {
     return monitoringHost;
   }
 
+  /**
+   * Returns the GCP project ID for client-side metrics export. Returns the explicit value if set
+   * via {@link Builder#setMetricsProjectId}, otherwise falls back to {@link #getProjectId()}.
+   */
+  String resolveMetricsProjectId() {
+    return metricsProjectId != null ? metricsProjectId : getProjectId();
+  }
+
   public TransactionOptions getDefaultTransactionOptions() {
     return defaultTransactionOptions;
   }