Enhanced Gauge API (#106)

stats.go

@@ -118,6 +118,15 @@ func (g *gauge) Update(v float64) {
 	atomic.StoreUint64(&g.updated, 1)
 }
 
+func (g *gauge) Add(v float64) {
+	v += math.Float64frombits(atomic.LoadUint64(&g.curr))
+	g.Update(v)
+}
+
+func (g *gauge) Sub(v float64) {
+	g.Add(-v)
+}
+
 func (g *gauge) value() float64 {
 	return math.Float64frombits(atomic.LoadUint64(&g.curr))
 }

stats_test.go

@@ -108,6 +108,14 @@ func TestGauge(t *testing.T) {
 	gauge.report("", nil, r)
 	assert.Equal(t, float64(42), r.last)
 
+	gauge.Add(4.2)
+	gauge.report("", nil, r)
+	assert.Equal(t, float64(46.2), r.last)
+
+	gauge.Sub(-4)
+	gauge.report("", nil, r)
+	assert.Equal(t, float64(50.2), r.last)
+
 	gauge.Update(1234)
 	gauge.Update(5678)
 	gauge.report("", nil, r)

types.go

@@ -69,6 +69,12 @@ type Counter interface {
 type Gauge interface {
 	// Update sets the gauges absolute value.
 	Update(value float64)
+
+	// Add updates gauges absolute value by adding 'value' to current value
+	Add(value float64)
+
+	// Sub updates gauges absolute value by subtracting 'value' from current value
+	Sub(value float64)
 }
 
 // Timer is the interface for emitting timer metrics.