[Fizz] Export debug function for FB (facebook#24024)

* export debug function for FB

* silence flow

packages/react-server-dom-relay/src/ReactDOMServerFB.js

@@ -92,4 +92,17 @@ function hasFinished(stream: Stream): boolean {
   return stream.destination.done;
 }
 
-export {renderToStream, renderNextChunk, hasFinished, abortStream};
+function debug(stream: Stream): any {
+  // convert to any to silence flow errors from opaque type
+  const request = (stream.request: any);
+  return {
+    pendingRootTasks: request.pendingRootTasks,
+    clientRenderedBoundaries: request.clientRenderedBoundaries.length,
+    completedBoundaries: request.completedBoundaries.length,
+    partialBoundaries: request.partialBoundaries.length,
+    allPendingTasks: request.allPendingTasks,
+    pingedTasks: request.pingedTasks.length,
+  };
+}
+
+export {renderToStream, renderNextChunk, hasFinished, abortStream, debug};