fix: fix channel priming by adding missing halfClose() (#9737)

Author: igorbernstein2

bigtable/bigtable-proxy/src/main/java/com/google/cloud/bigtable/examples/proxy/channelpool/DataChannel.java

@@ -201,6 +201,7 @@ public void onClose(Status status, Metadata trailers) {
         },
         metadata);
     call.sendMessage(request);
+    call.halfClose();
     call.request(Integer.MAX_VALUE);
 
     return f;