[EventEngine] Disable the backup poller if all EventEngine experiments are running (#37951)

Closes #37951

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37951 from drfloob:disable-backup-poller 1ce9d2d1d6
PiperOrigin-RevId: 688263398

src/core/client_channel/backup_poller.cc

@@ -57,8 +57,19 @@ static backup_poller* g_poller = nullptr;  // guarded by g_poller_mu
 // treated as const.
 static grpc_core::Duration g_poll_interval =
     grpc_core::Duration::Milliseconds(DEFAULT_POLL_INTERVAL_MS);
+// TODO(hork): delete the backup poller when EventEngine is rolled out
+// everywhere.
+static bool g_backup_polling_disabled;
 
 void grpc_client_channel_global_init_backup_polling() {
+  // Disable backup polling if EventEngine is used everywhere.
+  g_backup_polling_disabled = grpc_core::IsEventEngineClientEnabled() &&
+                              grpc_core::IsEventEngineListenerEnabled() &&
+                              grpc_core::IsEventEngineDnsEnabled();
+  if (g_backup_polling_disabled) {
+    return;
+  }
+
   gpr_mu_init(&g_poller_mu);
   int32_t poll_interval_ms =
       grpc_core::ConfigVars::Get().ClientChannelBackupPollIntervalMs();
@@ -146,7 +157,8 @@ static void g_poller_init_locked() {
 
 void grpc_client_channel_start_backup_polling(
     grpc_pollset_set* interested_parties) {
-  if (g_poll_interval == grpc_core::Duration::Zero() ||
+  if (g_backup_polling_disabled ||
+      g_poll_interval == grpc_core::Duration::Zero() ||
       grpc_iomgr_run_in_background()) {
     return;
   }
@@ -165,7 +177,8 @@ void grpc_client_channel_start_backup_polling(
 
 void grpc_client_channel_stop_backup_polling(
     grpc_pollset_set* interested_parties) {
-  if (g_poll_interval == grpc_core::Duration::Zero() ||
+  if (g_backup_polling_disabled ||
+      g_poll_interval == grpc_core::Duration::Zero() ||
       grpc_iomgr_run_in_background()) {
     return;
   }