add graphite bridge that support delta counters

vendor/github.com/prometheus/common/model/time.go

@@ -163,9 +163,21 @@ func (t *Time) UnmarshalJSON(b []byte) error {
 // This type should not propagate beyond the scope of input/output processing.
 type Duration time.Duration
 
+// Set implements pflag/flag.Value
+func (d *Duration) Set(s string) error {
+	var err error
+	*d, err = ParseDuration(s)
+	return err
+}
+
+// Type implements pflag.Value
+func (d *Duration) Type() string {
+	return "duration"
+}
+
 var durationRE = regexp.MustCompile("^([0-9]+)(y|w|d|h|m|s|ms)$")
 
-// StringToDuration parses a string into a time.Duration, assuming that a year
+// ParseDuration parses a string into a time.Duration, assuming that a year
 // always has 365d, a week always has 7d, and a day always has 24h.
 func ParseDuration(durationStr string) (Duration, error) {
 	matches := durationRE.FindStringSubmatch(durationStr)