Add GaugeRelative to support Telegraf statsd implementation

Telegraf doesn't support support negative counters, requiring relative gauge values instead. This adds the new method GaugeRelative to allow for this.

conn.go

@@ -1,6 +1,7 @@
 package statsd
 
 import (
+	"fmt"
 	"io"
 	"math/rand"
 	"net"
@@ -93,6 +94,20 @@ func (c *conn) metric(prefix, bucket string, n interface{}, typ string, rate flo
 	c.mu.Unlock()
 }
 
+func (c *conn) gaugeRelative(prefix, bucket string, value interface{}, tags string) {
+	c.mu.Lock()
+	l := len(c.buf)
+	c.appendBucket(prefix, bucket, tags)
+	if isNegative(value) {
+		c.appendNumber(value)
+	} else {
+		c.appendString(fmt.Sprintf("+%v", value))
+	}
+	c.appendType("g")
+	c.flushIfBufferFull(l)
+	c.mu.Unlock()
+}
+
 func (c *conn) gauge(prefix, bucket string, value interface{}, tags string) {
 	c.mu.Lock()
 	l := len(c.buf)

statsd.go

@@ -100,6 +100,14 @@ func (c *Client) Gauge(bucket string, value interface{}) {
 	c.conn.gauge(c.prefix, bucket, value, c.tags)
 }
 
+// GaugeRelative records an relative value for the given bucket.
+func (c *Client) GaugeRelative(bucket string, value interface{}) {
+	if c.skip() {
+		return
+	}
+	c.conn.gaugeRelative(c.prefix, bucket, value, c.tags)
+}
+
 // Timing sends a timing value to a bucket.
 func (c *Client) Timing(bucket string, value interface{}) {
 	if c.skip() {