docs: kafka message header limitation

[robcarlan-datadog]

This is in the DSM docs, but also adding it to the python documentation too.

DataDog/documentation#28871

Checklist

• PR author has checked that all the criteria below are met
• The PR description includes an overview of the change
• The PR description articulates the motivation for the change
• The change includes tests OR the PR description describes a testing strategy
• The PR description notes risks associated with the change, if any
• Newly-added code is easy to change
• The change follows the library release note guidelines
• The change includes or references documentation updates if necessary
• Backport labels are set (if applicable)

Reviewer Checklist

• Reviewer has checked that all the criteria below are met
• Title is accurate
• All changes are related to the pull request's stated goal
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Newly-added code is easy to change
• Release note makes sense to a user of the library
• If necessary, author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
• Backport labels are set in a manner that is consistent with the release branch maintenance policy

[github-actions]

CODEOWNERS have been resolved as:

ddtrace/contrib/_kafka.py                                               @DataDog/apm-core-python @DataDog/apm-idm-python

[github-actions]

Bootstrap import analysis

Comparison of import times between this PR and base.

Summary

The average import time from this PR is: 236 ± 3 ms.

The average import time from base is: 237 ± 4 ms.

The import time difference between this PR and base is: -1.1 ± 0.2 ms.

Import time breakdown

The following import paths have shrunk:

ddtrace.auto 1.945 ms (0.83%)

ddtrace.bootstrap.sitecustomize 1.273 ms (0.54%)

ddtrace.bootstrap.preload 1.273 ms (0.54%)

ddtrace.internal.products 1.273 ms (0.54%)

ddtrace.internal.remoteconfig.client 0.628 ms (0.27%)

ddtrace 0.672 ms (0.29%)

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-04-24 01:57:08

Comparing candidate commit bac7dfc in PR branch rob.carlan/update-kafka-integration-dsm-docs with baseline commit 064f872 in branch main.

Found 0 performance improvements and 4 performance regressions! Performance is the same for 476 metrics, 4 unstable metrics.

scenario:iast_aspects-ospathnormcase_aspect

• 🟥 execution_time [+283.457ns; +404.681ns] or [+8.313%; +11.868%]

scenario:iast_aspects-ospathsplit_aspect

• 🟥 execution_time [+758.479ns; +903.801ns] or [+15.877%; +18.919%]

scenario:iast_aspects-ospathsplitdrive_aspect

• 🟥 execution_time [+396.374ns; +500.117ns] or [+10.843%; +13.681%]

scenario:telemetryaddmetric-1-gauge-metric-1-times

• 🟥 execution_time [+165.408ns; +248.360ns] or [+7.939%; +11.920%]