If you still have a moment, a few small tweaks and complete unit test coverage, and I'd be happy to accept. (You should be able to access reviewable for replies to specific comments). Please let me know when I should take another look.

Reviewed 3 of 6 files at r1, 1 of 3 files at r2.
Reviewable status: 0 of 1 LGTMs obtained

alerts/handler.go line 184 at r2 (raw file):

			msgBody, err := rh.formatIssueBody(msg)
			if err != nil {
				return fmt.Errorf("format body for %q: %s", msg.GroupKey, err)

Is it possible to get unit test coverage for this error condition?

alerts/handler_test.go line 247 at r2 (raw file):

				},
			},
			titleTmpl: 	   `{{ (index .Data.Alerts 1).Status }}`,

Please run go fmt on all files.

alerts/template_test.go line 62 at r2 (raw file):

			body, err := rh.formatIssueBody(&msg)
			if tc.expectErrTxt == "" && err != nil {

Please simplify the conditional logic verifying the test; I see this is like the one below (it's on me for approving originally! I'll fix it after this change) but I find both hard to follow.

I recommend the (slightly modified) pattern auto-generated by VSCode's "Generate Unit Tests" for this function in Go.

Whether there was an error is sufficient; inspecting the error string content is not necessary imo. We know it will be due to the template execution by inspection.

For example:

func Test_formatIssueBody(t *testing.T) {
	tests := []struct {
		name     string
		template string
		want     string
		wantErr  bool
	}{
		{
			name:     "success",
			template: "foo",
			want:     "foo",
			wantErr:  false,
		},
	}
	for _, tt := range tests {
		t.Run(tt.name, func(t *testing.T) {
			got, err := formatIssueBody(tt.args.msg)
			if (err != nil) != tt.wantErr {
				t.Errorf("formatIssueBody() error = %v, wantErr %v", err, tt.wantErr)
				return
			}
			if got != tt.want {
				t.Errorf("formatIssueBody() = %v, want %v", got, tt.want)
			}
		})
	}
}