Add the rlb dispatcher flush

Special API that is C-FFI-like exported from the RLB later.

glean-core/src/lib.rs

@@ -438,6 +438,26 @@ pub fn persist_ping_lifetime_data() {
     });
 }
 
+/// Unblock the global dispatcher to start processing queued tasks.
+pub extern "C" fn rlb_flush_dispatcher() {
+    let was_initialized = was_initialize_called();
+
+    // Panic in debug mode
+    debug_assert!(!was_initialized);
+
+    // In release do a check and bail out
+    if was_initialized {
+        log::error!(
+            "Tried to flush the dispatcher from outside, but Glean was initialized in the RLB."
+        );
+        return;
+    }
+
+    if let Err(err) = dispatcher::flush_init() {
+        log::error!("Unable to flush the preinit queue: {}", err);
+    }
+}
+
 fn initialize_core_metrics(glean: &Glean, client_info: &ClientInfoMetrics) {
     core_metrics::internal_metrics::app_build.set_sync(glean, &client_info.app_build[..]);
     core_metrics::internal_metrics::app_display_version