Added default first hist bucket if needed (#532)

Author: yonatang

lib/histogram.go

@@ -68,11 +68,15 @@ func (bs *Buckets) UnmarshalText(value []byte) error {
 	if len(value) < 2 || value[0] != '[' || value[len(value)-1] != ']' {
 		return fmt.Errorf("bad buckets: %s", value)
 	}
-	for _, v := range strings.Split(string(value[1:len(value)-1]), ",") {
+	for i, v := range strings.Split(string(value[1:len(value)-1]), ",") {
 		d, err := time.ParseDuration(strings.TrimSpace(v))
 		if err != nil {
 			return err
 		}
+		// add a default range of [0-Buckets[0]) if needed
+		if i == 0 && d > 0 {
+			*bs = append(*bs, 0)
+		}
 		*bs = append(*bs, d)
 	}
 	if len(*bs) == 0 {

lib/histogram_test.go

@@ -57,6 +57,7 @@ func TestBuckets_UnmarshalText(t *testing.T) {
 		"[0,5ms]":             {0, 5 * time.Millisecond},
 		"[0, 5ms]":            {0, 5 * time.Millisecond},
 		"[   0,5ms, 10m    ]": {0, 5 * time.Millisecond, 10 * time.Minute},
+		"[3ms,10ms]":          {0, 3 * time.Millisecond, 10 * time.Millisecond},
 	} {
 		var got Buckets
 		if err := got.UnmarshalText([]byte(value)); err != nil {