Migrate fuzzer to use shiny new metrics2 package

go/metrics2/counter.go

Index: go/metrics2/counter.go
diff --git a/go/metrics2/counter.go b/go/metrics2/counter.go
index 15750a505c317504e447f778a2d0be1059cf7afa..3752e2c291cb24008f70ed601e4f582c35afb377 100644
--- a/go/metrics2/counter.go
+++ b/go/metrics2/counter.go
@@ -51,3 +51,10 @@ func (c *Counter) Reset() {
 	defer c.mtx.Unlock()
 	c.m.Update(0)
 }
+
+// Get returns the current value in the counter.
+func (c *Counter) Get() int64 {
+	c.mtx.Lock()
+	defer c.mtx.Unlock()
+	return c.m.Get()
+}