Move bucket index calculations to the place where they are used

This can help https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1096

histogram.go

@@ -86,6 +86,13 @@ func (h *Histogram) Update(v float64) {
 		return
 	}
 	bucketIdx := (math.Log10(v) - e10Min) * bucketsPerDecimal
+	h.mu.Lock()
+	h.sum += v
+	if bucketIdx < 0 {
+		h.lower++
+	} else if bucketIdx >= bucketsCount {
+		h.upper++
+	} else {
 		idx := uint(bucketIdx)
 		if bucketIdx == float64(idx) {
 			// Edge case for 10^n values, which must go to the lower bucket
@@ -94,13 +101,6 @@ func (h *Histogram) Update(v float64) {
 		}
 		decimalBucketIdx := idx / bucketsPerDecimal
 		offset := idx % bucketsPerDecimal
-	h.mu.Lock()
-	h.sum += v
-	if bucketIdx < 0 {
-		h.lower++
-	} else if bucketIdx >= bucketsCount {
-		h.upper++
-	} else {
 		db := h.decimalBuckets[decimalBucketIdx]
 		if db == nil {
 			var b [bucketsPerDecimal]uint64