Expose prometheus_tsdb_start_time_seconds metric

Signed-off-by: Bob Shannon <bshannon@palantir.com>

db.go

@@ -123,6 +123,7 @@ type dbMetrics struct {
 	compactionsTriggered prometheus.Counter
 	cutoffs              prometheus.Counter
 	cutoffsFailed        prometheus.Counter
+	startTime            prometheus.Gauge
 	tombCleanTimer       prometheus.Histogram
 }
 
@@ -157,6 +158,10 @@ func newDBMetrics(db *DB, r prometheus.Registerer) *dbMetrics {
 		Name: "prometheus_tsdb_retention_cutoffs_failures_total",
 		Help: "Number of times the database failed to cut off block data from disk.",
 	})
+	m.startTime = prometheus.NewGauge(prometheus.GaugeOpts{
+		Name: "prometheus_tsdb_start_time_seconds",
+		Help: "Oldest timestamp stored in the database.",
+	})
 	m.tombCleanTimer = prometheus.NewHistogram(prometheus.HistogramOpts{
 		Name: "prometheus_tsdb_tombstone_cleanup_seconds",
 		Help: "The time taken to recompact blocks to remove tombstones.",
@@ -170,6 +175,7 @@ func newDBMetrics(db *DB, r prometheus.Registerer) *dbMetrics {
 			m.cutoffs,
 			m.cutoffsFailed,
 			m.compactionsTriggered,
+			m.startTime,
 			m.tombCleanTimer,
 		)
 	}
@@ -238,6 +244,9 @@ func Open(dir string, l log.Logger, r prometheus.Registerer, opts *Options) (db
 
 	go db.run()
 
+	head := db.Head()
+	db.metrics.startTime.Set(float64(head.minTime))
+
 	return db, nil
 }