LGTM, thanks.

@Alhanaqtah your code changes look good but as @colega pointed out we're missing a unit test. I can give you some pointers.

• func TestHeadAppender_AppendCT(t *testing.T) {

  prometheus/tsdb/head_test.go

  Line 6499 in b07b552

  func TestHeadAppender_AppendCT(t *testing.T) {

  is a good example, you can duplicate this test to test that errors are handled properly.
• Originally the appendableHistogram functions did not return other types of errors other than ErrOutOfOrderSample this why we got to this situation. However now they can return ErrDuplicateSampleForTimestamp ErrTooOldSample ErrOutOfBounds and ErrOutOfOrderSample. So for example I think that if you attempt two AppendHistogramCTZeroSample calls with the same values you should be getting ErrDuplicateSampleForTimestamp from appendableHistogram and your new return clauses should be executed.

Are you up for this quick test?

Thanks, I checked that the test panics without the fix.

LGTM. I'd prefer a few more testcases, but I can add mine later.

Thank you for the PR and the tests.