promql: fix smoothed rate returning zero for data only after range

For smoothed rate/increase, a result should only be returned
when there is data available to interpolate across the range. If the
range has a single data point only on one side, the result is
meaningless and should be empty.

The "data only before" case was already handled: if the last fetched
sample is at or before rangeStart, extendedRate returns nothing.

Add the symmetric guard for the "data only after" case: if the first
fetched sample is strictly after rangeEnd, return nothing as well.

This mirrors the behaviour described in prometheus/prometheus#18295,
where a smoothed rate that has no data before the range should not
return zero.

Signed-off-by: Julien Pivotto <291750+roidelapluie@users.noreply.github.com>

promql/functions.go

@@ -142,6 +142,9 @@ func extendedRate(vals Matrix, args parser.Expressions, enh *EvalNodeHelper, isC
 	if f[lastSampleIndex].T <= rangeStart {
 		return enh.Out, annos
 	}
+	if smoothed && f[firstSampleIndex].T > rangeEnd {
+		return enh.Out, annos
+	}
 
 	left := pickOrInterpolateLeft(f, firstSampleIndex, rangeStart, smoothed, isCounter)
 	right := pickOrInterpolateRight(f, lastSampleIndex, rangeEnd, smoothed, isCounter)

promql/promqltest/testdata/extended_vectors.test

@@ -417,4 +417,20 @@ eval instant at 45s withreset smoothed
     withreset 3
 
 eval instant at 30s notregular smoothed
-    notregular 2
+    notregular 2
+
+# Smoothed rate returns empty when data is only before or only after the range.
+clear
+load 10s
+    metric _ 5+1x4
+
+eval instant at 5s rate(metric[5s] smoothed)
+
+eval instant at 5s increase(metric[5s] smoothed)
+
+eval instant at 15s increase(metric[10s] smoothed)
+    {} 0.5
+
+eval instant at 60s rate(metric[5s] smoothed)
+
+eval instant at 60s increase(metric[5s] smoothed)