ci: add auto-commit of rustfmt changes action to CI APMSP-2770 (#1548)

# What does this PR do?

Introduces a new GH workflow that uses [rustfmt-check](https://github.com/mbrobbel/rustfmt-check) and can automatically apply `rustfmt` changes via a commit on your PR.

This is currently opt-in via the `commit-rustfmt-changes` label. 

The existing validation workflow still exists and is required to pass. 

# Motivation

What inspired you to submit this pull request?

# Additional Notes

Anything else we should know when reviewing?

# How to test the change?

Describe here in detail how the change can be validated.


Co-authored-by: edmund.kump <edmund.kump@datadoghq.com>

Author: ekump

.github/workflows/all-checks.yml

@@ -16,4 +16,4 @@ jobs:
                 delay: '3'
                 retries: '45'
                 polling_interval: '1'
-                checks_exclude: 'devflow/merge,dd-gitlab/default-pipeline'
+                checks_exclude: 'devflow/merge,dd-gitlab/default-pipeline,Rustfmt Auto / rustfmt'

.github/workflows/rustfmt-auto.yml

@@ -0,0 +1,31 @@
+name: Rustfmt Auto
+on:
+  pull_request:
+    types: [opened, synchronize, reopened, labeled]
+
+jobs:
+  rustfmt:
+    runs-on: ubuntu-latest
+    if: >-
+      contains(github.event.pull_request.labels.*.name, 'commit-rustfmt-changes') &&
+      !startsWith(github.head_ref, 'mq-working-branch')
+    permissions:
+      id-token: write
+      contents: write
+    steps:
+      - name: Checkout sources
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # 4.2.2
+        with:
+          ref: ${{ github.head_ref }}
+      - name: Get GitHub App token
+        uses: DataDog/dd-octo-sts-action@acaa02eee7e3bb0839e4272dacb37b8f3b58ba80 # v1.0.3
+        id: octo-sts
+        with:
+          scope: DataDog/libdatadog
+          policy: self.write.rustfmt
+      - name: Install nightly-2026-02-08 toolchain and rustfmt
+        run: rustup install nightly-2026-02-08 && rustup default nightly-2026-02-08 && rustup component add rustfmt
+      - name: Run rustfmt and commit changes
+        uses: mbrobbel/rustfmt-check@e7ac5685995bd39fb14f6c83fa6f10627e92e680 # master
+        with:
+          token: ${{ steps.octo-sts.outputs.token }}

CONTRIBUTING.md

@@ -78,6 +78,18 @@ We also recommend that you share in your description:
 If at any point you have a question or need assistance with your pull request, feel free to mention a project member!
 We're always happy to help contributors with their pull requests.
 
+## Code Formatting
+
+All Rust code must be formatted with `rustfmt` using the project's configuration in `rustfmt.toml`. You can format your
+code locally by running:
+
+```bash
+cargo +nightly fmt --all
+```
+
+If you'd like CI to automatically format your code and commit the changes to your PR, add the `commit-rustfmt-changes`
+label to your pull request. This will trigger an automatic formatting commit if any changes are needed.
+
 ## Commit Message Guidelines
 
 This project uses [Conventional Commits](https://www.conventionalcommits.org/) for commit messages and pull request titles.