simplify gauge "+"/"-": float64 naturally saturates, negative is OK

Gauge relative change limit checks are a hold-over from when
gauge values were uint64.

float64 loses much precision at very high values, and it rounds to
values it is able to represent: MaxFloat64 + 123456789.0 = MaxFloat64
(you have to subtract about 1e292 to get it to budge!)

Negative gauge results seem to be part of the informal spec:
> This implies you can't explicitly set a gauge to a negative number
> without first setting it to zero.
https://github.com/etsy/statsd/blob/master/docs/metric_types.md#gauges

And really, who uses relative gauge changes anyway ...

statsdaemon.go

@@ -149,26 +149,14 @@ func packetHandler(s *Packet) {
 		// if missing gets nil []float64, works with append()
 		timers[s.Bucket] = append(timers[s.Bucket], s.ValFlt)
 	case "g":
-		gaugeValue, _ := gauges[s.Bucket]
-
+		var gaugeValue float64
 		if s.ValStr == "" {
 			gaugeValue = s.ValFlt
 		} else if s.ValStr == "+" {
-			// watch out for overflows
-			if s.ValFlt > (math.MaxFloat64 - gaugeValue) {
-				gaugeValue = math.MaxFloat64
-			} else {
-				gaugeValue += s.ValFlt
-			}
+			gaugeValue = gauges[s.Bucket] + s.ValFlt
 		} else if s.ValStr == "-" {
-			// subtract checking for negative numbers
-			if s.ValFlt > gaugeValue {
-				gaugeValue = 0
-			} else {
-				gaugeValue -= s.ValFlt
-			}
+			gaugeValue = gauges[s.Bucket] - s.ValFlt
 		}
-
 		gauges[s.Bucket] = gaugeValue
 	case "c":
 		counters[s.Bucket] += s.ValFlt / float64(s.Sampling)

statsdaemon_test.go

@@ -433,14 +433,14 @@ func TestPacketHandlerGauge(t *testing.T) {
 	packetHandler(p)
 	assert.Equal(t, gauges["gaugor"], float64(327))
 
-	// <0 overflow
+	// going negative
 	p.ValFlt = 10
 	p.ValStr = ""
 	packetHandler(p)
 	p.ValFlt = 20
 	p.ValStr = "-"
 	packetHandler(p)
-	assert.Equal(t, gauges["gaugor"], float64(0))
+	assert.Equal(t, gauges["gaugor"], float64(-10))
 
 	// >MaxFloat64 overflow
 	p.ValFlt = float64(math.MaxFloat64 - 10)