lib/streamaggr: ignore out of order samples for last output

This is a follow-up for 6a465f6e29

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/pull/5931

docs/stream-aggregation.md

@@ -19,10 +19,10 @@ The aggregation is applied to all the metrics received via any [supported data i
 and/or scraped from [Prometheus-compatible targets](https://docs.victoriametrics.com/#how-to-scrape-prometheus-exporters-such-as-node-exporter)
 after applying all the configured [relabeling stages](https://docs.victoriametrics.com/vmagent.html#relabeling).
 
-Stream aggregation ignores timestamps associated with the input [samples](https://docs.victoriametrics.com/keyConcepts.html#raw-samples).
-It expects that the ingested samples have timestamps close to the current time.
+By default stream aggregation ignores timestamps associated with the input [samples](https://docs.victoriametrics.com/keyConcepts.html#raw-samples).
+It expects that the ingested samples have timestamps close to the current time. See [how to ignore old samples](#ignoring-old-samples).
 
-Stream aggregation is configured via the following command-line flags:
+Stream aggregation can be configured via the following command-line flags:
 
 - `-remoteWrite.streamAggr.config` at [vmagent](https://docs.victoriametrics.com/vmagent.html).
   This flag can be specified individually per each `-remoteWrite.url`.

lib/streamaggr/last.go

@@ -12,9 +12,10 @@ type lastAggrState struct {
 }
 
 type lastStateValue struct {
-	mu      sync.Mutex
-	last    float64
-	deleted bool
+	mu        sync.Mutex
+	last      float64
+	timestamp int64
+	deleted   bool
 }
 
 func newLastAggrState() *lastAggrState {
@@ -31,7 +32,8 @@ func (as *lastAggrState) pushSamples(samples []pushSample) {
 		if !ok {
 			// The entry is missing in the map. Try creating it.
 			v = &lastStateValue{
-				last: s.value,
+				last:      s.value,
+				timestamp: s.timestamp,
 			}
 			vNew, loaded := as.m.LoadOrStore(outputKey, v)
 			if !loaded {
@@ -45,7 +47,10 @@ func (as *lastAggrState) pushSamples(samples []pushSample) {
 		sv.mu.Lock()
 		deleted := sv.deleted
 		if !deleted {
-			sv.last = s.value
+			if s.timestamp >= sv.timestamp {
+				sv.last = s.value
+				sv.timestamp = s.timestamp
+			}
 		}
 		sv.mu.Unlock()
 		if deleted {

lib/streamaggr/streamaggr_test.go

@@ -254,13 +254,14 @@ func TestAggregatorsSuccess(t *testing.T) {
   outputs: [count_samples, sum_samples, count_series, last]
 `, `
 foo{abc="123"} 4
-bar 5
+bar 5 100
+bar 34 10
 foo{abc="123"} 8.5
 foo{abc="456",de="fg"} 8
-`, `bar:1m_count_samples 1
+`, `bar:1m_count_samples 2
 bar:1m_count_series 1
 bar:1m_last 5
-bar:1m_sum_samples 5
+bar:1m_sum_samples 39
 foo:1m_count_samples{abc="123"} 2
 foo:1m_count_samples{abc="456",de="fg"} 1
 foo:1m_count_series{abc="123"} 1
@@ -269,7 +270,7 @@ foo:1m_last{abc="123"} 8.5
 foo:1m_last{abc="456",de="fg"} 8
 foo:1m_sum_samples{abc="123"} 12.5
 foo:1m_sum_samples{abc="456",de="fg"} 8
-`, "1111")
+`, "11111")
 
 	// Special case: __name__ in `by` list - this is the same as empty `by` list
 	f(`