(#67) Attempt to build on AppEngine

runtime.go

@@ -119,7 +119,7 @@ func CaptureRuntimeMemStatsOnce(r Registry) {
 	runtimeMetrics.MemStats.StackSys.Update(int64(memStats.StackSys))
 	runtimeMetrics.MemStats.Sys.Update(int64(memStats.Sys))
 	runtimeMetrics.MemStats.TotalAlloc.Update(int64(memStats.TotalAlloc))
-	runtimeMetrics.NumCgoCall.Update(int64(runtime.NumCgoCall()))
+	runtimeMetrics.NumCgoCall.Update(numCgoCall())
 	runtimeMetrics.NumGoroutine.Update(int64(runtime.NumGoroutine()))
 }
 

runtime_cgo.go

@@ -0,0 +1,9 @@
+// +build cgo
+
+package metrics
+
+import "runtime"
+
+func numCgoCall() int64 {
+	return runtime.NumCgoCall()
+}

runtime_no_cgo.go

@@ -0,0 +1,7 @@
+// +build !cgo
+
+package metrics
+
+func numCgoCall() int64 {
+	return 0
+}