Assume counters start at zero after reset.

promql/functions.go

@@ -68,7 +68,7 @@ func extrapolatedRate(ev *evaluator, arg Expr, isCounter bool, isRate bool) mode
 		for _, sample := range samples.Values {
 			currentValue := sample.Value
 			if isCounter && currentValue < lastValue {
-				counterCorrection += lastValue - currentValue
+				counterCorrection += lastValue
 			}
 			lastValue = currentValue
 		}

promql/testdata/functions.test

@@ -69,6 +69,18 @@ eval instant at 50m increase(http_requests[100m])
 
 clear
 
+# Test for increase() with counter reset.
+# When the counter is reset, it always starts at 0.
+# So the sequence 3 2 (decreasing counter = reset) is interpreted the same as 3 0 1 2.
+# Prometheus assumes it missed the intermediate values 0 and 1.
+load 5m
+	http_requests{path="/foo"}	0 1 2 3 2 3 4
+
+eval instant at 30m increase(http_requests[30m])
+    {path="/foo"} 7
+
+clear
+
 # Tests for irate().
 load 5m
 	http_requests{path="/foo"}	0+10x10