properly handle Histogram.Update(1e-9)

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1096

histogram.go

@@ -94,7 +94,7 @@ func (h *Histogram) Update(v float64) {
 		h.upper++
 	} else {
 		idx := uint(bucketIdx)
-		if bucketIdx == float64(idx) {
+		if bucketIdx == float64(idx) && idx > 0 {
 			// Edge case for 10^n values, which must go to the lower bucket
 			// according to Prometheus logic for `le`-based histograms.
 			idx--

histogram_test.go

@@ -97,6 +97,8 @@ prefix_count 120
 	h.Update(math.Inf(-1))
 	h.Update(math.NaN())
 	h.Update(-123)
+	// See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1096
+	h.Update(math.Float64frombits(0x3e112e0be826d695))
 
 	// Make sure the histogram becomes visible in the output of WritePrometheus,
 	// since now it contains values.