Support comma-delimited string as labels in issue template (#21831) (#21873)

Backport #21831.

The [labels in issue YAML templates](https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/syntax-for-issue-forms#top-level-syntax)
can be a string array or a comma-delimited string, so a single string
should be valid labels.

The old codes committed in #20987 ignore this, that's why the warning is
displayed:

<img width="618" alt="image" src="https://user-images.githubusercontent.com/9418365/202112642-93dc72d0-71c3-40a2-9720-30fc2d48c97c.png">

Fixes #17877.

modules/structs/issue_test.go

@@ -8,6 +8,7 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+	"gopkg.in/yaml.v3"
 )
 
 func TestIssueTemplate_Type(t *testing.T) {
@@ -41,3 +42,65 @@ func TestIssueTemplate_Type(t *testing.T) {
 		})
 	}
 }
+
+func TestIssueTemplateLabels_UnmarshalYAML(t *testing.T) {
+	tests := []struct {
+		name    string
+		content string
+		tmpl    *IssueTemplate
+		want    *IssueTemplate
+		wantErr string
+	}{
+		{
+			name:    "array",
+			content: `labels: ["a", "b", "c"]`,
+			tmpl: &IssueTemplate{
+				Labels: []string{"should_be_overwrote"},
+			},
+			want: &IssueTemplate{
+				Labels: []string{"a", "b", "c"},
+			},
+		},
+		{
+			name:    "string",
+			content: `labels: "a,b,c"`,
+			tmpl: &IssueTemplate{
+				Labels: []string{"should_be_overwrote"},
+			},
+			want: &IssueTemplate{
+				Labels: []string{"a", "b", "c"},
+			},
+		},
+		{
+			name:    "empty",
+			content: `labels:`,
+			tmpl: &IssueTemplate{
+				Labels: []string{"should_be_overwrote"},
+			},
+			want: &IssueTemplate{
+				Labels: nil,
+			},
+		},
+		{
+			name: "error",
+			content: `
+labels:
+  a: aa
+  b: bb
+`,
+			tmpl:    &IssueTemplate{},
+			wantErr: "line 3: cannot unmarshal !!map into IssueTemplateLabels",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := yaml.Unmarshal([]byte(tt.content), tt.tmpl)
+			if tt.wantErr != "" {
+				assert.EqualError(t, err, tt.wantErr)
+			} else {
+				assert.NoError(t, err)
+				assert.Equal(t, tt.want, tt.tmpl)
+			}
+		})
+	}
+}