[8.x](backport #42327) x-pack/filebeat/input: Fix truncation of bodies in request tracing

[mergify]

Proposed commit message

x-pack/filebeat/input: Fix truncation of bodies in request tracing

When logging request traces, truncate the request/response body to 10%
of the maximum log file size.

Previously, bodies were truncated to the maximum file size, less 10kB.
10kB is a reasonable number for the other trace details, but space is
also required for encoding the body data as a JSON string value.

One example JSON body was 15% larger after encoding, but the 10kB
margin is 1% or less of the total limit. A body approaching the size
limit would typically generate a log entry that exceeded the limit.

Truncating large log entries to fit the file size limit means there may
only be one such entry per file. By truncating body data to 10% of the
file limit, we can expect to see entries for several request/response
pairs in each file.

The default maximum file size of 1MB gives a default maximum body size
of 100kB.

The behavior of request tracing for the HTTP Endpoint input is
unchanged: it always truncates request bodies to a size of 10kiB.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Related issues

• Relates [Filebeat] http trace logger - truncate response bodies larger than request.tracer.maxsize #37826

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This is an automatic backport of pull request #42327 done by [Mergify](https://mergify.com).

[elasticmachine]

Pinging @elastic/security-service-integrations (Team:Security-Service Integrations)