counter.go: add Counter.AddInt64() method

This method can be used for avoiding int overflows.

counter.go

@@ -42,6 +42,11 @@ func (c *Counter) Add(n int) {
 	atomic.AddUint64(&c.n, uint64(n))
 }
 
+// AddInt64 adds n to c.
+func (c *Counter) AddInt64(n int64) {
+	atomic.AddUint64(&c.n, uint64(n))
+}
+
 // Get returns the current value for c.
 func (c *Counter) Get() uint64 {
 	return atomic.LoadUint64(&c.n)