request help: upstream related header is missing when using external plugins

### Description

Hi,

I am using `ext-plugin-post-req`, `ext-plugin-post-resp` and sending logs to loki using `loki-logger`. However, when i enabled external plugins, upstream related header is missing in logs.

It [seems](https://github.com/apache/apisix/blob/master/apisix/utils/log-util.lua#L151) that upstream related variables is using nginx variable. I tried to log them in the external plugin code but it returns nothing.

TestFilter
```java
@Override
public void filter(HttpRequest request, HttpResponse response, PluginFilterChain chain) {
LOG.error("[TestFilter - postReq]" + NginxVars.UPSTREAM_RESPONSE_TIME.getValue() + ": " + request.getVars(NginxVars.UPSTREAM_RESPONSE_TIME.getValue()));
LOG.error("[TestFilter - postReq]" + NginxVars.UPSTREAM_ADDR.getValue() + ": " + request.getVars(NginxVars.UPSTREAM_ADDR.getValue()));
LOG.error("[TestFilter - postReq]" + NginxVars.REQUEST_URI.getValue() + ": " + request.getVars(NginxVars.REQUEST_URI.getValue()));
chain.filter(request, response);
}

@Override
public void postFilter(PostRequest request, PostResponse response, PluginFilterChain chain) {
response.setHeader(NginxVars.UPSTREAM_ADDR.getValue(),
request.getVars(NginxVars.UPSTREAM_ADDR.getValue()));
response.setHeader(NginxVars.UPSTREAM_RESPONSE_TIME.getValue(),
request.getVars(NginxVars.UPSTREAM_RESPONSE_TIME.getValue()));

LOG.error("[TestFilter - postRes]" + NginxVars.UPSTREAM_RESPONSE_TIME.getValue() + ": " + request.getVars(NginxVars.UPSTREAM_RESPONSE_TIME.getValue()));
LOG.error("[TestFilter - postRes]" + NginxVars.UPSTREAM_ADDR.getValue() + ": " + request.getVars(NginxVars.UPSTREAM_ADDR.getValue()));
LOG.error("[TestFilter - postRes]" + NginxVars.REQUEST_URI.getValue() + ": " + request.getVars(NginxVars.REQUEST_URI.getValue()));
chain.postFilter(request, response);
}

@Override
public List requiredVars() {
List vars = new ArrayList<>();
vars.add(NginxVars.HTTP_ACCEPT.getValue());
vars.add(NginxVars.REQUEST_URI.getValue());
vars.add(NginxVars.HTTP_OPENAPI_ORIGINAL_ACCEPT.getValue());
vars.add(NginxVars.HTTP_OPENAPIKEY.getValue());
vars.add(NginxVars.UPSTREAM_RESPONSE_TIME.getValue());
vars.add(NginxVars.UPSTREAM_ADDR.getValue());
return vars;
}
```
Plugins Logs
```
2023-10-23 01:53:43,844 ERROR epollEventLoopGroup-2-5 - [TestFilter - postReq]upstream_response_time:
2023-10-23 01:53:43,844 ERROR epollEventLoopGroup-2-5 - [TestFilter - postReq]upstream_addr:
2023-10-23 01:53:43,844 ERROR epollEventLoopGroup-2-5 - [TestFilter - postReq]request_uri: /rest/.../...(omit for security)
2023-10-23 01:53:43,862 ERROR epollEventLoopGroup-2-5 - [TestFilter - postRes]upstream_response_time:
2023-10-23 01:53:43,862 ERROR epollEventLoopGroup-2-5 - [TestFilter - postRes]upstream_addr:
2023-10-23 01:53:43,863 ERROR epollEventLoopGroup-2-5 - [TestFilter - postRes]request_uri: /rest/../...(omit for security)
```

Loki (without external plugins)

hide IP for security reason

Loki (with external plugins)

Here's my plugin configuration
```
plugins:
- config:
client_headers:
...
request_headers:
...
request_method: GET
upstream_headers:
...
uri: '...'
enable: true
name: forward-auth
- config:
disable: false
prefer_name: true
enable: true
name: prometheus
- config:
endpoint_addrs:
- >-
http://loki-loki-distributed-gateway.monitor.svc.cluster.local:80
include_req_body: true
include_req_body_expr:
- - http_response_status
- ~*
- '200'
include_resp_body: true
include_resp_body_expr:
- - http_response_status
- ~*
- '200'
tenant_id: dev-openapi-logging
enable: true
name: loki-logger
- config:
headers:
...
uri: ...
enable: true
name: proxy-rewrite
- config:
conf:
- name: TestFilter
value: |
test
enable: true
name: ext-plugin-post-req
- config:
conf:
- name: TestFilter
value: |
test
enable: true
name: ext-plugin-post-resp
```

Could you help me to get upstream related headers?
Thanks,

### Environment

- APISIX version (run `apisix version`):
```
apisix version
/usr/local/openresty//luajit/bin/luajit /usr/local/apisix/apisix/cli/apisix.lua version
3.4.0
```
- Operating system (run `uname -a`):
```
uname -a
Linux dev-apisix-56c7dc6b9-m2mfx 5.10.192-183.736.amzn2.x86_64 #1 SMP Wed Sep 6 21:15:41 UTC 2023 x86_64 GNU/Linux
```
- OpenResty / Nginx version (run `openresty -V` or `nginx -V`):
- etcd version, if relevant (run `curl http://127.0.0.1:9090/v1/server_info`):
- APISIX Dashboard version, if relevant:
- Plugin runner version, for issues related to plugin runners: apisix-java-plugin-runner 0.4.0
- LuaRocks version, for installation issues (run `luarocks --version`):