fix: move mismatch log to client creation path, avoid log spam

Address review feedback:
- Move project mismatch log inside the new-client branch so it
  fires at most once per DatabaseId, not on every getDatabaseClient
- equals/hashCode: SpannerOptions inherits from ServiceOptions and
  does not override these methods; monitoringHost follows the same
  pattern

Author: RRap0so

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

@@ -296,19 +296,6 @@ public InstanceAdminClient getInstanceAdminClient() {
   public DatabaseClient getDatabaseClient(DatabaseId db) {
     synchronized (this) {
       checkClosed();
-      SpannerOptions opts = getOptions();
-      String metricsProject = opts.resolveMetricsProjectId();
-      if (opts.isEnableBuiltInMetrics()
-          && metricsProject != null
-          && !metricsProject.equals(db.getInstanceId().getProject())) {
-        logger.log(
-            Level.WARNING,
-            "DatabaseId project ''{0}'' differs from the project used for client-side metrics"
-                + " export ''{1}''. Metrics will be exported to ''{1}'', which may cause"
-                + " permission errors. Set SpannerOptions.Builder.setMetricsProjectId(\"{0}\")"
-                + " to fix this.",
-            new Object[] {db.getInstanceId().getProject(), metricsProject});
-      }
       String clientId = null;
       if (dbClients.containsKey(db) && !dbClients.get(db).isValid()) {
         // Close the invalidated client and remove it.
@@ -319,6 +306,17 @@ public DatabaseClient getDatabaseClient(DatabaseId db) {
       if (dbClients.containsKey(db)) {
         return dbClients.get(db);
       } else {
+        SpannerOptions opts = getOptions();
+        String metricsProject = opts.resolveMetricsProjectId();
+        if (opts.isEnableBuiltInMetrics()
+            && metricsProject != null
+            && !metricsProject.equals(db.getInstanceId().getProject())) {
+          logger.log(
+              Level.INFO,
+              "Built-in metrics project ''{0}'' differs from DatabaseId project ''{1}''."
+                  + " Use setMetricsProjectId() to override if needed.",
+              new Object[] {metricsProject, db.getInstanceId().getProject()});
+        }
         if (clientId == null) {
           clientId = nextDatabaseClientId(db);
         }