xds: buffer responses during ext-proc request draining

When the external processor stream state is DRAINING, response headers,
status, and trailers received from the upstream server should not be
processed or forwarded immediately.

Modify ExternalProcessorClientInterceptor to buffer these response
headers and status/trailers in savedHeaders, savedStatus, and
savedTrailers, returning early when in DRAINING state. These will be
flushed and delivered to the application call listener once the sidecar
stream completes.

Author: kannanjgithub

xds/src/main/java/io/grpc/xds/ExternalProcessorClientInterceptor.java

@@ -1200,6 +1200,10 @@ public void onHeaders(Metadata headers) {
       System.out.println("=== CLIENT RECEIVED RESPONSE HEADERS: " + headers + " ===");
       dataPlaneClientCall.serverHeadersStartNanos = System.nanoTime();
       responseHeadersSent.set(true);
+      if (dataPlaneClientCall.extProcStreamState.get().isDraining()) {
+        this.savedHeaders = headers;
+        return;
+      }
       boolean sendResponseHeaders =
           dataPlaneClientCall.currentProcessingMode.getResponseHeaderMode()
               == ProcessingMode.HeaderSendMode.SEND
@@ -1284,6 +1288,10 @@ public void onClose(Status status, Metadata trailers) {
         return;
       }
 
+      if (dataPlaneClientCall.extProcStreamState.get().isDraining()) {
+        return;
+      }
+
       if (!responseHeadersSent.get()) {
         trailersOnly.set(true);
       }