Add metrics for isolation.

db.go

@@ -131,6 +131,8 @@ type DB struct {
 
 type dbMetrics struct {
 	loadedBlocks         prometheus.GaugeFunc
+	lowWatermark         prometheus.GaugeFunc
+	highWatermark        prometheus.GaugeFunc
 	reloads              prometheus.Counter
 	reloadsFailed        prometheus.Counter
 	compactionsTriggered prometheus.Counter
@@ -150,6 +152,20 @@ func newDBMetrics(db *DB, r prometheus.Registerer) *dbMetrics {
 		defer db.mtx.RUnlock()
 		return float64(len(db.blocks))
 	})
+	m.lowWatermark = prometheus.NewGaugeFunc(prometheus.GaugeOpts{
+		Name: "tsdb_isolation_low_watermark",
+		Help: "The lowest write id that is still referenced.",
+	}, func() float64 {
+		return float64(db.readLowWatermark())
+	})
+	m.highWatermark = prometheus.NewGaugeFunc(prometheus.GaugeOpts{
+		Name: "tsdb_isolation_high_watermark",
+		Help: "The highest write id that has been given out.",
+	}, func() float64 {
+		db.writeMtx.Lock()
+		defer db.writeMtx.Unlock()
+		return float64(db.writeLastId)
+	})
 	m.reloads = prometheus.NewCounter(prometheus.CounterOpts{
 		Name: "prometheus_tsdb_reloads_total",
 		Help: "Number of times the database reloaded block data from disk.",
@@ -178,6 +194,8 @@ func newDBMetrics(db *DB, r prometheus.Registerer) *dbMetrics {
 	if r != nil {
 		r.MustRegister(
 			m.loadedBlocks,
+			m.lowWatermark,
+			m.highWatermark,
 			m.reloads,
 			m.reloadsFailed,
 			m.cutoffs,