add unit tests with all negative values for histogram_stddev and var

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

promql/engine_test.go

@@ -3498,6 +3498,38 @@ func TestNativeHistogram_HistogramStdDevVar(t *testing.T) {
 			},
 			stdVar: 1544.8582535368798, // actual variance: 1738.4082
 		},
+		{
+			name: "-100000, -10000, -1000, -888, -888, -100, -50, -9, -8, -3",
+			h: &histogram.Histogram{
+				Count:     10,
+				ZeroCount: 0,
+				Sum:       -112946,
+				Schema:    0,
+				NegativeSpans: []histogram.Span{
+					{Offset: 2, Length: 3},
+					{Offset: 1, Length: 2},
+					{Offset: 2, Length: 1},
+					{Offset: 3, Length: 1},
+					{Offset: 2, Length: 1},
+				},
+				NegativeBuckets: []int64{1, 0, 0, 0, 0, 2, -2, 0},
+			},
+			stdVar: 1240930974.5260057, // actual variance: 882690990
+		},
+		{
+			name: "-10 x10",
+			h: &histogram.Histogram{
+				Count:     10,
+				ZeroCount: 0,
+				Sum:       -100,
+				Schema:    0,
+				NegativeSpans: []histogram.Span{
+					{Offset: 4, Length: 1},
+				},
+				NegativeBuckets: []int64{10},
+			},
+			stdVar: 454.2741699796952, // actual variance: 0
+		},
 		{
 			name: "-50, -8, 0, 3, 8, 9, 100, NaN",
 			h: &histogram.Histogram{