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

[chrisberkhout]

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.

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

[elasticmachine]

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

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @chrisberkhout? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label and remove the backport-8.x label.

[andrewkroh]

That's an excellent commit message. Thank you.

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

I find this behavior a bit surprising. I would have expected that if I needed to capture a full 5 MiB response body that I would need to increase the max size to something a little larger than 5 MiB, but not 10x the size. What do others think? At a minimum, I think we need to mention this behavior in the documentation associated with the tracer settings.

[andrewkroh]

I think it would be nice to include a bit of the "how" part, like ... "by limiting bodies to 10% of the maximum file size".