Reset avg accumulator correctly (#541)

Signed-off-by: 🌲 Harry 🌊 John 🏔 <[email protected]>

Author: harry671003

engine/engine_test.go

@@ -325,6 +325,15 @@ or
 			step:  30 * time.Second,
 			query: `avg without (pod) (http_requests_total)`,
 		},
+		{
+			name: "avg fuzz",
+			load: `load 30s
+			    http_requests_total{pod="nginx-1", route="/"} NaN NaN NaN NaN NaN 0.000053234+0.000003x10 NaN NaN
+			    http_requests_total{pod="nginx-2", route="/"} NaN NaN NaN NaN NaN 0.00004123412+0.000004x10 NaN NaN`,
+			query: `avg(stdvar_over_time(http_requests_total[2m:1m]))`,
+			start: time.Unix(0, 0),
+			end:   time.Unix(300, 0),
+		},
 		{
 			name: "func with scalar arg that selects storage, checks whether same series handled correctly",
 			load: `load 30s

execution/aggregate/accumulator.go

@@ -473,6 +473,9 @@ func (a *avgAcc) ValueType() ValueType {
 
 func (a *avgAcc) Reset(_ float64) {
 	a.hasValue = false
+	a.incremental = false
+	a.kahanSum = 0
+	a.kahanC = 0
 	a.count = 0
 
 	a.histCount = 0