some more fixes

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

Author: amanycodes

promql/functions.go

@@ -1351,11 +1351,12 @@ func histogramVariance(vals []parser.Value, enh *EvalNodeHelper, varianceToResul
 			}
 			var val float64
 			switch {
-			case bucket.Lower <= 0 && bucket.Upper >= 0:
-				val = 0
 			case sample.H.UsesCustomBuckets():
 				// Use arithmetic mean in case of custom buckets.
-				val = (bucket.Upper * bucket.Lower) / 2.0
+				val = (bucket.Upper + bucket.Lower) / 2.0
+			case bucket.Lower <= 0 && bucket.Upper >= 0:
+				// Use zero (effectively the arithmetic mean) in the zero bucket of a standard exponential histogram.
+				val = 0
 			default:
 				// Use geometric mean in case of standard exponential buckets.
 				val = math.Sqrt(bucket.Upper * bucket.Lower)

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"} 2.962188994031886
-	{start="negative"} 3.9825070621406313
+	{start="positive"} 2.7435461458749795
+	{start="negative"} 4.187667907081458
 
 # Test histogram_stdvar. This has no classic equivalent.
 eval instant at 50m histogram_stdvar(testhistogram3)
-	{start="positive"} 8.774563636363636
-	{start="negative"} 15.860362500000003
+	{start="positive"} 7.527045454545455
+	{start="negative"} 17.5365625
 
 # Test histogram_fraction.
 #