fixed tests and logic

Signed-off-by: amanycodes <amanycodes@gmail.com>

Author: amanycodes

promql/functions.go

@@ -1351,12 +1351,12 @@ func histogramVariance(vals []parser.Value, enh *EvalNodeHelper, varianceToResul
 				continue
 			}
 			var val float64
-			if bucket.Lower <= 0 && bucket.Upper >= 0 {
-				val = 0
+			if sample.H.UsesCustomBuckets() {
+				// use arithmetic mean in case of NHCB.
+				val = (bucket.Upper * bucket.Lower) / 2.0
 			} else {
-				if sample.H.UsesCustomBuckets() {
-					// use arithmetic mean in case of NHCB.
-					val = (bucket.Upper * bucket.Lower) / 2.0
+				if bucket.Lower <= 0 && bucket.Upper >= 0 {
+					val = 0
 				} else {
 					val = math.Sqrt(bucket.Upper * bucket.Lower)
 					if bucket.Upper < 0 {

promql/promqltest/testdata/histograms.test

@@ -95,13 +95,13 @@ eval instant at 50m histogram_avg(testhistogram3)
 
 # Test histogram_stddev. This has no classic equivalent.
 eval instant at 50m histogram_stddev(testhistogram3)
-	{start="positive"} 2.768081055040247
-	{start="negative"} 4.187667907081458
+	{start="positive"} 2.962188994031886
+	{start="negative"} 3.9825070621406313
 
 # Test histogram_stdvar. This has no classic equivalent.
 eval instant at 50m histogram_stdvar(testhistogram3)
-	{start="positive"} 7.662272727272727
-	{start="negative"} 17.536562500000002
+	{start="positive"} 8.774563636363636
+	{start="negative"} 15.860362500000003
 
 # Test histogram_fraction.