Fixes: Won't compile on AppEngine #67

runtime_cgo.go

@@ -1,4 +1,5 @@
 // +build cgo
+// +build !appengine
 
 package metrics
 

runtime_no_cgo.go

@@ -1,4 +1,4 @@
-// +build !cgo
+// +build !cgo appengine
 
 package metrics