improve handling of negative bounds in histogram std dev/var

Signed-off-by: Jeanette Tan <jeanette.tan@grafana.com>

promql/engine_test.go

@@ -3496,7 +3496,7 @@ func TestNativeHistogram_HistogramStdDevVar(t *testing.T) {
 				},
 				NegativeBuckets: []int64{1, 0},
 			},
-			stdVar: 1544.8582535368798, // actual variance: 1738.4082
+			stdVar: 1844.4651144196398, // actual variance: 1738.4082
 		},
 		{
 			name: "-100000, -10000, -1000, -888, -888, -100, -50, -9, -8, -3",
@@ -3514,7 +3514,7 @@ func TestNativeHistogram_HistogramStdDevVar(t *testing.T) {
 				},
 				NegativeBuckets: []int64{1, 0, 0, 0, 0, 2, -2, 0},
 			},
-			stdVar: 1240930974.5260057, // actual variance: 882690990
+			stdVar: 759352122.1939945, // actual variance: 882690990
 		},
 		{
 			name: "-10 x10",
@@ -3528,7 +3528,7 @@ func TestNativeHistogram_HistogramStdDevVar(t *testing.T) {
 				},
 				NegativeBuckets: []int64{10},
 			},
-			stdVar: 454.2741699796952, // actual variance: 0
+			stdVar: 1.725830020304794, // actual variance: 0
 		},
 		{
 			name: "-50, -8, 0, 3, 8, 9, 100, NaN",

promql/functions.go

@@ -1116,6 +1116,9 @@ func funcHistogramStdDev(vals []parser.Value, args parser.Expressions, enh *Eval
 				val = 0
 			} else {
 				val = math.Sqrt(bucket.Upper * bucket.Lower)
+				if bucket.Upper < 0 {
+					val = -val
+				}
 			}
 			delta := val - mean
 			variance, cVariance = kahanSumInc(bucket.Count*delta*delta, variance, cVariance)
@@ -1149,6 +1152,9 @@ func funcHistogramStdVar(vals []parser.Value, args parser.Expressions, enh *Eval
 				val = 0
 			} else {
 				val = math.Sqrt(bucket.Upper * bucket.Lower)
+				if bucket.Upper < 0 {
+					val = -val
+				}
 			}
 			delta := val - mean
 			variance, cVariance = kahanSumInc(bucket.Count*delta*delta, variance, cVariance)