remove reallocs and overallocs for s.lset in bucket.go (#1718)

- append to s.lset caused reallocs and overallocs
- this is now fixed by properly setting capacity on s.lset
- pprof shows "-316.22MB, 5.00% of 6322.82MB total" for sample data set and query I used

Signed-off-by: Philip Panyukov <ppanyukov@googlemail.com>

pkg/store/bucket.go

@@ -680,7 +680,7 @@ func blockSeries(
 			return nil, nil, errors.Wrap(err, "read series")
 		}
 		s := seriesEntry{
-			lset: make([]storepb.Label, 0, len(lset)),
+			lset: make([]storepb.Label, 0, len(lset)+len(extLset)),
 			refs: make([]uint64, 0, len(chks)),
 			chks: make([]storepb.AggrChunk, 0, len(chks)),
 		}