headIndexReader.LabelNamesFor: skip not found series

It's quite common during the compaction cycle to hold series IDs for
series that aren't in the TSDB head anymore.

We shouldn't fail if that happens, as the caller has no way to figure
out which one of the IDs doesn't exist.

Fixes https://github.com/prometheus/prometheus/issues/14278

Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>

tsdb/head_read.go

@@ -277,7 +277,9 @@ func (h *headIndexReader) LabelNamesFor(ctx context.Context, ids ...storage.Seri
 		}
 		memSeries := h.head.series.getByID(chunks.HeadSeriesRef(id))
 		if memSeries == nil {
-			return nil, storage.ErrNotFound
+			// Series not found, this happens during compaction,
+			// when series was garbage collected after the caller got the series IDs.
+			continue
 		}
 		memSeries.lset.Range(func(lbl labels.Label) {
 			namesMap[lbl.Name] = struct{}{}