fix: Set upstream and upstream_latency vars in ext-plugin-post-resp for logging

[MariaLapovska]

Description

When using ext-plugin-post-resp and http-logger, some values are missing from the logs, specifically the ones related to upstream - upstream and upstream_latency.
This is happening because upstream_response_time and upstream_addr nginx vars are unavailable, so when log-util.lua is trying to inject them into logs, no value is present.
This PR adds these two vars to ext-plugin-post-resp, they are now set explicitly after the call to upstream.

Which issue(s) this PR fixes:

Fixes #13016

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[janiussyafiq]

would you be able to add test case that cover your fix?

[MariaLapovska]

@janiussyafiq I'm not sure which test case I can add for this, since it simply adds two vars to nginx context. Would you be able to help me out?

[shreemaan-abhishek]

@MariaLapovska you should add tests that verify that the variables you newly introduced are actually taking effect and hold correct values.

[Baoyuantop]

Hi @MariaLapovska, following up on the previous review comments. Please let us know if you have any updates. Thank you.

[moonming]

Hi @MariaLapovska, thank you for fixing the missing upstream variables in ext-plugin-post-resp!

Setting upstream and upstream_latency vars in the ext-plugin-post-resp phase for logging is a valid fix — these variables should be available for downstream loggers to capture.

One requirement: This PR needs test cases to verify:

1. The upstream variable is populated correctly after ext-plugin-post-resp runs
2. The upstream_latency variable reflects the actual upstream response time
3. Logger plugins (e.g., http-logger) correctly capture these values when used alongside ext-plugin-post-resp

The fix is only 5 lines, so adding focused test cases should be straightforward. Looking forward to the update!