Accessing request headers in envoy_on_response (lua HTTP filter).

I'm trying to use a [lua HTTP filter](https://www.envoyproxy.io/docs/envoy/latest/configuration/http_filters/lua_filter) to set a response header whose value depends on a request header.

As a simple example, suppose we want to copy a header from the request to the response. How can we do this using a lua filter?

Maybe we can store the request header in a "global" table during `envoy_on_request` and read it later when `envoy_on_response` is called. However, even if this approach makes sense, how can we correlate responses with requests to be able to do this?