Expose prometheus_tsdb_lowest_timestamp metric (#363)

* Expose prometheus_tsdb_start_time_seconds metric

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

* Search for block with smallest minTime

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

* PR comments

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

* PR comment: Make metric name more accurate

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

db.go

@@ -125,6 +125,7 @@ type dbMetrics struct {
 	compactionsTriggered prometheus.Counter
 	cutoffs              prometheus.Counter
 	cutoffsFailed        prometheus.Counter
+	startTime            prometheus.GaugeFunc
 	tombCleanTimer       prometheus.Histogram
 }
 
@@ -172,6 +173,17 @@ 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.NewGaugeFunc(prometheus.GaugeOpts{
+		Name: "prometheus_tsdb_lowest_timestamp",
+		Help: "Lowest timestamp value stored in the database.",
+	}, func() float64 {
+		db.mtx.RLock()
+		defer db.mtx.RUnlock()
+		if len(db.blocks) == 0 {
+			return float64(db.head.minTime)
+		}
+		return float64(db.blocks[0].meta.MinTime)
+	})
 	m.tombCleanTimer = prometheus.NewHistogram(prometheus.HistogramOpts{
 		Name: "prometheus_tsdb_tombstone_cleanup_seconds",
 		Help: "The time taken to recompact blocks to remove tombstones.",
@@ -186,6 +198,7 @@ func newDBMetrics(db *DB, r prometheus.Registerer) *dbMetrics {
 			m.cutoffs,
 			m.cutoffsFailed,
 			m.compactionsTriggered,
+			m.startTime,
 			m.tombCleanTimer,
 		)
 	}