Merge pull request #17320 from prometheus/beorn7/histogram3

model/histogram: Fix checkHistogramCustomBounds to accept -Inf

model/histogram/generic.go

@@ -468,11 +468,11 @@ func checkHistogramBuckets[BC BucketCount, IBC InternalBucketCount](buckets []IB
 
 func checkHistogramCustomBounds(bounds []float64, spans []Span, numBuckets int) error {
 	prev := math.Inf(-1)
-	for _, curr := range bounds {
+	for i, curr := range bounds {
 		if math.IsNaN(curr) {
 			return ErrHistogramCustomBucketsNaN
 		}
-		if curr <= prev {
+		if i > 0 && curr <= prev {
 			return fmt.Errorf("previous bound is %f and current is %f: %w", prev, curr, ErrHistogramCustomBucketsInvalid)
 		}
 		prev = curr

model/histogram/histogram_test.go

@@ -1579,6 +1579,12 @@ func TestHistogramValidation(t *testing.T) {
 			},
 			errMsg: "custom buckets: last +Inf bound must not be explicitly defined: histogram custom bounds must be finite",
 		},
+		"valid custom buckets histogram with explicit -Inf bound": {
+			h: &Histogram{
+				Schema:       CustomBucketsSchema,
+				CustomValues: []float64{math.Inf(-1), 1},
+			},
+		},
 		"reject custom buckets histogram with NaN bound": {
 			h: &Histogram{
 				Schema:       CustomBucketsSchema,