service::client_state: add a const version of get_trace_state()

tracing::begin() requires a non-const version, tracing::trace()
requires a const version.

Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>

service/client_state.hh

@@ -118,6 +118,10 @@ public:
         return _trace_state_ptr;
     }
 
+    const tracing::trace_state_ptr& get_trace_state() const {
+        return _trace_state_ptr;
+    }
+
     lw_shared_ptr<utils::UUID>& tracing_session_id_ptr() {
         return _tracing_session_id;
     }