Add an auto-labeller to always benchmark lock file changes (#5763)

* Add an auto-labeller to always benchmark lock file changes.

* What's New entry.

.github/labeler.yml

@@ -0,0 +1,6 @@
+# Make sure any dependency changes are benchmarked (only changes to the locks
+#  make a material difference - changes to the Conda YAML files are not
+#  benchmarked).
+benchmark_this:
+- changed-files:
+  - any-glob-to-any-file: 'requirements/locks/*.lock'

.github/workflows/labeler.yml

@@ -0,0 +1,15 @@
+# Reference
+#   - https://github.com/actions/labeler
+
+name: "Pull Request Labeler"
+on:
+- pull_request_target
+
+jobs:
+  labeler:
+    permissions:
+      contents: read
+      pull-requests: write
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/labeler@v5

docs/src/whatsnew/latest.rst

@@ -72,7 +72,11 @@ This document explains the changes made to Iris for this release
 💼 Internal
 ===========
 
-#. N/A
+#. `@trexfeathers`_ used the `Pull Request Labeler Github action`_ to add the
+   ``benchmark_this`` label (:ref:`more info <on_demand_pr_benchmark>`) to
+   pull requests that modify ``requirements/locks/*.lock`` files - ensuring
+   that we know whether dependency changes will affect performance.
+   (:pull:`5763`)
 
 
 .. comment
@@ -84,3 +88,5 @@ This document explains the changes made to Iris for this release
 
 .. comment
     Whatsnew resources in alphabetical order:
+
+.. _Pull Request Labeler GitHub action: https://github.com/actions/labeler