fix: use max_req_body_bytes field in custom_format

[Revolyssup]

Description

Problem description
The customer uses the kafka-logger plug-in and configures max_req_body_bytes and max_resp_body_bytes respectively, and the sizes of the request and response bodies exceed the configured values. However, in the log content actually uploaded to kafka, the response body is truncated, but the request body is not truncated. Perform truncation.

Reproduction steps
Configure plugin_metadata for kafka-logger and set a custom log format. The log format contains $request_body
Enable the kafka-logger plug-in in the global plug-in or route or service, and configure max_req_body_bytes and max_resp_body_bytes
The client initiates a request, the request body size is greater than max_req_body_bytes, and hits the kafka-logger plug-in
Viewing the log in kafka, you can see that the content of request_body is the complete request body and is not truncated.

Fix: Make sure the field is used in custom_format as well

Fixes # (issue)

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)