Adding a test for non-monotonic buckets

prometheus/histogram_test.go

@@ -119,6 +119,28 @@ func BenchmarkHistogramWrite8(b *testing.B) {
 	benchmarkHistogramWrite(8, b)
 }
 
+func TestHistogramNonMonotonicBuckets(t *testing.T) {
+	testCases := map[string][]float64{
+		"not strictly monotonic":  {1, 2, 2, 3},
+		"not monotonic at all":    {1, 2, 4, 3, 5},
+		"have +Inf in the middle": {1, 2, math.Inf(+1), 3},
+	}
+	for name, buckets := range testCases {
+		func() {
+			defer func() {
+				if r := recover(); r == nil {
+					t.Errorf("Buckets %v are %s but NewHistogram did not panic.", buckets, name)
+				}
+			}()
+			_ = NewHistogram(HistogramOpts{
+				Name:    "test_histogram",
+				Help:    "helpless",
+				Buckets: buckets,
+			})
+		}()
+	}
+}
+
 // Intentionally adding +Inf here to test if that case is handled correctly.
 // Also, getCumulativeCounts depends on it.
 var testBuckets = []float64{-2, -1, -0.5, 0, 0.5, 1, 2, math.Inf(+1)}