Allow access to call stack from call.

This API parallels the equivalent one in channel.

src/core/surface/call.c

@@ -981,3 +981,8 @@ void grpc_call_set_deadline(grpc_call_element *elem, gpr_timespec deadline) {
   call->have_alarm = 1;
   grpc_alarm_init(&call->alarm, deadline, call_alarm, call, gpr_now());
 }
+
+grpc_call_stack *grpc_call_get_call_stack(grpc_call *call) {
+  return CALL_STACK_FROM_CALL(call);
+}
+

src/core/surface/call.h

@@ -64,6 +64,8 @@ void grpc_call_client_initial_metadata_complete(
 void grpc_call_set_deadline(grpc_call_element *surface_element,
                             gpr_timespec deadline);
 
+grpc_call_stack *grpc_call_get_call_stack(grpc_call *call);
+
 /* Given the top call_element, get the call object. */
 grpc_call *grpc_call_from_top_element(grpc_call_element *surface_element);