Bot: Label docs-only changes no-changelog

Docs-only changes usually do not require a changelog entry.
Automatically add the `no-changelog` label for docs-only changes so docs
authors do not receive unnecessary notifications about missing
`no-changelog` labels and workflow failures for docs-only pull requests.

bot/internal/bot/label.go

@@ -66,6 +66,11 @@ func (b *Bot) labels(ctx context.Context, files []github.PullRequestFile) ([]str
 		labels = append(labels, string(prSize(files)))
 	}
 
+	if !requiresChangelog(files) {
+		log.Println("Label: Adding no-changelog.")
+		labels = append(labels, "no-changelog")
+	}
+
 	// The branch name is unsafe, but here we are simply adding a label.
 	if b.c.Environment.Repository != env.CloudRepo && isReleaseBranch(b.c.Environment.UnsafeBase) {
 		log.Println("Label: Found backport branch.")
@@ -102,6 +107,17 @@ func deduplicate(s []string) []string {
 	return out
 }
 
+// requiresChangelog indicates whether, based on the files in a pull request,
+// the PR should have the "no-changelog" label.
+func requiresChangelog(files []github.PullRequestFile) bool {
+	for _, f := range files {
+		if !strings.HasPrefix(f.Name, "docs/") {
+			return true
+		}
+	}
+	return false
+}
+
 var prefixes = map[string]map[string][]string{
 	env.TeleportRepo: {
 		"bpf/":                {"bpf"},

bot/internal/bot/label_test.go

@@ -56,7 +56,7 @@ func TestLabel(t *testing.T) {
 					Deletions: 10,
 				},
 			},
-			labels: []string{"documentation", string(small)},
+			labels: []string{"documentation", "no-changelog", string(small)},
 		},
 		{
 			desc:   "helm",
@@ -91,7 +91,7 @@ func TestLabel(t *testing.T) {
 					Deletions: 1000,
 				},
 			},
-			labels: []string{"backport", "documentation", string(xlarge)},
+			labels: []string{"backport", "documentation", "no-changelog", string(xlarge)},
 		},
 		{
 			desc:   "web only",