compact: relax index label checking

It is possible that Prometheus will produce time series with duplicate
labels. E.g.: a{b="2",b="2"} 1

Where such metric is pushed to Prometheus either wittingly or
unwittingly, the compact process will stall because the check will not
pass. For example:

{"caller":"main.go:181","err":"error executing compaction: compaction failed: compaction: gather index issues for block /data/compact/0@{monitor="monitor",replica="repl"}/01D34EDQMSQ29RHAC47XGKHGC7: out-of-order label set {**name**="foo",exported_job="vv",host="172_16_226_56",host="172_16_226_56",region="lt",subtask_index="5",task_attempt_id="32e4b047bb768583ff57c709be3b1046",task_attempt_num="8",task_id="688c028a219ff3372f3eecb0ee5811f9",task_name="Source:_foo",tenant="abc",tier="cooltier",tm_id="53b2ed987b08f427dec4ee1465df91fa"} for series 2594231","level":"error","msg":"running command failed","ts":"2019-02-11T13:30:33.901722306Z"}

This commit fixes the mentioned issue. Amend the e2e test to test the
case when a series has identical labels.

pkg/block/index.go

@@ -347,7 +347,7 @@ func GatherIndexIssueStats(logger log.Logger, fn string, minTime int64, maxTime
 		}
 		l0 := lset[0]
 		for _, l := range lset[1:] {
-			if l.Name <= l0.Name {
+			if l.Name < l0.Name {
 				return stats, errors.Errorf("out-of-order label set %s for series %d", lset, id)
 			}
 			l0 = l

pkg/compact/compact_e2e_test.go

@@ -180,7 +180,7 @@ func TestGroup_Compact_e2e(t *testing.T) {
 		extLset := labels.Labels{{Name: "e1", Value: "1"}}
 		b1, err := testutil.CreateBlock(prepareDir, []labels.Labels{
 			{{Name: "a", Value: "1"}},
-			{{Name: "a", Value: "2"}},
+			{{Name: "a", Value: "2"}, {Name: "a", Value: "2"}},
 			{{Name: "a", Value: "3"}},
 			{{Name: "a", Value: "4"}},
 		}, 100, 0, 1000, extLset, 124)