don't block on rendering status pages

runners/google-cloud-dataflow-java/worker/src/main/java/org/apache/beam/runners/dataflow/worker/windmill/client/AbstractWindmillStream.java

@@ -276,6 +276,7 @@ public final void maybeSendHealthCheck(Instant lastSendThreshold) {
    *     information. Blocking sends are made beneath this stream object's lock which could block
    *     status page rendering.
    */
+  @SuppressWarnings("GuardedBy")
   public final void appendSummaryHtml(PrintWriter writer) {
     appendSpecificHtml(writer);
     debugMetrics.printRestartsHtml(writer);