also ngx_http_upstream_response_time_variable and ngx_http_upstream_response_length_variable, + 2 for separator

if (state[i].peer) {
*p++ = ',';
*p++ = ' ';

} else {
*p++ = ' ';
*p++ = ':';
*p++ = ' ';

if (++i == r->upstream_states->nelts) {
break;
}

continue;
}

it can be 3 bytes, right?