bug: built-in variables are not supported in _meta.filter

[bzp2010]

Current Behavior

The filters in the plugin support only Nginx variables, not APISIX's built-in variables.

For example, I can use the Nginx variable http_xxx, but I can't use APISIX custom variables like arg_xxxx or graphql_xxxx.

Expected Behavior

It can support all APISIX variable syntax, including cookie_xxxx, arg_xxxx, post_arg_xxxx, graphql_xxxx, etc.

Error Logs

No response

Steps to Reproduce

1. Set the filter in the plugin to use the above unsupported syntax.

---

The reason is that we didn't pass in our ctx for that expr executor, and the expr uses its default ngx.var, which wouldn't support extended syntax.

https://github.com/apache/apisix/blob/master/apisix/plugin.lua#L437
https://github.com/api7/lua-resty-expr/blob/main/lib/resty/expr/v1.lua#L376

We just need to change it to ok, err = ex:eval(ctx.var) and it will support those extended syntaxes.

Environment

• APISIX version (run apisix version): 3.0.0

[soulbird]

I will fix it later.