feat(log): add custom variables for enhanced logging capabilities

[felpasl]

Description

Introduced a comprehensive set of custom logging variables as functions, including:

• latency: Total request latency in milliseconds
• upstream_latency: Time spent communicating with upstream servers
• apisix_latency: APISIX processing time (total latency minus upstream latency)
• client_ip: Remote client IP address
• start_time: Request start time in milliseconds
• hostname: Server hostname
• version: APISIX version
• request_method: HTTP method
• request_headers: Request headers
• request_querystring: Query parameters
• request_body: Request body (with size limits)
• response_status: HTTP response status
• response_headers: Response headers
• response_size: Response size in bytes
• response_body: Response body
• upstream: Upstream server address
• url: Full request URL
• consumer_username: Consumer username
• service_id: Associated service ID
• route_id: Associated route ID

Simplified the function to use the custom_vars table for dynamic variable resolution, replacing hardcoded logic for request_body with a generic approach.

Added group_id to the consumer object in full log entries.

Which issue(s) this PR fixes:

Fixes #

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)

[Baoyuantop]

We have some built-in variables and all nginx variables are supported. Can you explain in detail why this change is needed?

https://apisix.apache.org/docs/apisix/apisix-variable/
https://nginx.org/en/docs/varindex.html

[felpasl]

The goal here is to give users more freedom and control over how logs are handled.

Right now, the default log includes useful fields but it also bundles headers and other request/response data that I don’t want to store due to privacy concerns and storage overhead.

I want to be able to:

• Send audit logs to Loki
• Stream consumption data to Kafka

Customize log formats based on destination and purpose

To do that, I need those key fields (latency, hostname, method, version, client IP, etc.) to be available in log_format, so I can use them in custom_log without relying on the default log structure. This change enables that flexibility while keeping observability intact and avoiding unnecessary data.

Let me know if you have concerns or suggestions