message: build: mark a sometimes unused function parameter as unused
  *Description:*
  Before the change, an optimized build would fail, because the use of kvs is compiled out.
  *Risk Level:* Low
  Testing: Built debug and optimized
diff: @@ -230,6 +230,7 @@ void SubsetLoadBalancer::update(uint32_t priority, const HostVector& hosts_added
  },
  [&](LbSubsetEntryPtr entry, HostPredicate predicate, const SubsetMetadata& kvs,
  bool adding_host) {
  + UNREFERENCED_PARAMETER(kvs);
  if (adding_host) {
  ENVOY_LOG(debug, "subset lb: creating load balancer for {}",
  describeMetadata(kvs));
  