docs: add writeup of how autoinstrumentation works [LANGPLAT-352]

[emmettbutler]

This change adds a written description of the technical design of autoinstrumentation, for the purpose of making it easier to contribute changes to the library.

The new writing is in contributing-design.rst.

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:

docs/contributing-design.rst                                            @DataDog/python-guild
docs/contributing.rst                                                   @DataDog/python-guild
docs/spelling_wordlist.txt                                              @DataDog/python-guild

[github-actions]

Bootstrap import analysis

Comparison of import times between this PR and base.

Summary

The average import time from this PR is: 280 ± 3 ms.

The average import time from base is: 282 ± 3 ms.

The import time difference between this PR and base is: -2.3 ± 0.1 ms.

Import time breakdown

The following import paths have shrunk:

ddtrace.auto 2.019 ms (0.72%)

ddtrace.bootstrap.sitecustomize 1.336 ms (0.48%)

ddtrace.bootstrap.preload 1.336 ms (0.48%)

ddtrace.internal.remoteconfig.client 0.640 ms (0.23%)

ddtrace 0.683 ms (0.24%)

ddtrace.internal._unpatched 0.031 ms (0.01%)

json 0.031 ms (0.01%)

json.decoder 0.031 ms (0.01%)

re 0.031 ms (0.01%)

enum 0.031 ms (0.01%)

types 0.031 ms (0.01%)

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-06-11 15:19:05

Comparing candidate commit b7df5a7 in PR branch emmett.butler/design-docs with baseline commit e7c0d74 in branch main.

Found 0 performance improvements and 1 performance regressions! Performance is the same for 555 metrics, 6 unstable metrics.

scenario:iastaspects-format_map_aspect

• 🟥 execution_time [+425.129ns; +507.787ns] or [+13.121%; +15.672%]