small tsdb fixes (#6616)

* tsdb: register compactions_skipped_total

That metric was not registered.

I also reordered the metrics in the list.

* tsdb: display correct error when WAL can't be read

Signed-off-by: Julien Pivotto <roidelapluie@inuits.eu>

tsdb/db.go

@@ -155,15 +155,15 @@ type dbMetrics struct {
 	symbolTableSize      prometheus.GaugeFunc
 	reloads              prometheus.Counter
 	reloadsFailed        prometheus.Counter
-	compactionsTriggered prometheus.Counter
 	compactionsFailed    prometheus.Counter
-	timeRetentionCount   prometheus.Counter
+	compactionsTriggered prometheus.Counter
 	compactionsSkipped   prometheus.Counter
+	sizeRetentionCount   prometheus.Counter
+	timeRetentionCount   prometheus.Counter
 	startTime            prometheus.GaugeFunc
 	tombCleanTimer       prometheus.Histogram
 	blocksBytes          prometheus.Gauge
 	maxBytes             prometheus.Gauge
-	sizeRetentionCount   prometheus.Counter
 }
 
 func newDBMetrics(db *DB, r prometheus.Registerer) *dbMetrics {
@@ -248,14 +248,15 @@ func newDBMetrics(db *DB, r prometheus.Registerer) *dbMetrics {
 			m.symbolTableSize,
 			m.reloads,
 			m.reloadsFailed,
-			m.timeRetentionCount,
-			m.compactionsTriggered,
 			m.compactionsFailed,
+			m.compactionsTriggered,
+			m.compactionsSkipped,
+			m.sizeRetentionCount,
+			m.timeRetentionCount,
 			m.startTime,
 			m.tombCleanTimer,
 			m.blocksBytes,
 			m.maxBytes,
-			m.sizeRetentionCount,
 		)
 	}
 	return m
@@ -566,7 +567,7 @@ func Open(dir string, l log.Logger, r prometheus.Registerer, opts *Options) (db
 
 	if initErr := db.head.Init(minValidTime); initErr != nil {
 		db.head.metrics.walCorruptionsTotal.Inc()
-		level.Warn(db.logger).Log("msg", "encountered WAL read error, attempting repair", "err", err)
+		level.Warn(db.logger).Log("msg", "encountered WAL read error, attempting repair", "err", initErr)
 		if err := wlog.Repair(initErr); err != nil {
 			return nil, errors.Wrap(err, "repair corrupted WAL")
 		}