lib/storage: add metrics for calculating skipped rows outside the retention

The metrics are:

    - vm_too_big_timestamp_rows_total
    - vm_too_small_timestamp_rows_total

app/vmstorage/main.go

@@ -301,6 +301,13 @@ func registerStorageMetrics(strg *storage.Storage) {
 		return float64(idbm().SizeBytes)
 	})
 
+	metrics.NewGauge(`vm_too_big_timestamp_rows_total`, func() float64 {
+		return float64(m().TooBigTimestampRows)
+	})
+	metrics.NewGauge(`vm_too_small_timestamp_rows_total`, func() float64 {
+		return float64(m().TooSmallTimestampRows)
+	})
+
 	metrics.NewGauge(`vm_rows{type="storage/big"}`, func() float64 {
 		return float64(tm().BigRowsCount)
 	})

lib/storage/storage.go

@@ -65,6 +65,9 @@ type Storage struct {
 
 	currHourMetricIDsUpdaterWG sync.WaitGroup
 	retentionWatcherWG         sync.WaitGroup
+
+	tooSmallTimestampRows uint64
+	tooBigTimestampRows   uint64
 }
 
 // OpenStorage opens storage on the given path with the given number of retention months.
@@ -271,6 +274,9 @@ func (s *Storage) idb() *indexDB {
 
 // Metrics contains essential metrics for the Storage.
 type Metrics struct {
+	TooSmallTimestampRows uint64
+	TooBigTimestampRows   uint64
+
 	TSIDCacheSize       uint64
 	TSIDCacheSizeBytes  uint64
 	TSIDCacheRequests   uint64
@@ -308,6 +314,9 @@ func (m *Metrics) Reset() {
 
 // UpdateMetrics updates m with metrics from s.
 func (s *Storage) UpdateMetrics(m *Metrics) {
+	m.TooSmallTimestampRows += atomic.LoadUint64(&s.tooSmallTimestampRows)
+	m.TooBigTimestampRows += atomic.LoadUint64(&s.tooBigTimestampRows)
+
 	var cs fastcache.Stats
 	s.tsidCache.UpdateStats(&cs)
 	m.TSIDCacheSize += cs.EntriesCount
@@ -764,8 +773,14 @@ func (s *Storage) add(rows []rawRow, mrs []MetricRow, precisionBits uint8) ([]ra
 			// doesn't know how to work with them.
 			continue
 		}
-		if mr.Timestamp < minTimestamp || mr.Timestamp > maxTimestamp {
+		if mr.Timestamp < minTimestamp {
-			// Skip rows with timestamps outside the retention.
+			// Skip rows with too small timestamps outside the retention.
+			atomic.AddUint64(&s.tooSmallTimestampRows, 1)
+			continue
+		}
+		if mr.Timestamp > maxTimestamp {
+			// Skip rows with too big timestamps significantly exceeding the current time.
+			atomic.AddUint64(&s.tooBigTimestampRows, 1)
 			continue
 		}
 		r := &rows[rowsLen+j]