How to create new built-in variables of nginx on openresty?

[hui li]

Because when we use apisix, there are certain requirements for the format
of access.log, and the format needs to be similar to the following:
[$ http_host] [$ route_id] [$ upstream_id] [$ uri] [$ scheme] [rsp_st: $
status] [ups_st: $ upstream_status]
For example, route_id is the id of the route object. This kind of data is
not the default built-in variable of nginx. How can I create the built-in
variable nginx of route_id?