Add "filters/count" filter.

go/src/infra/gae/libs/gae/filters/count/count.go

Index: go/src/infra/gae/libs/gae/filters/count/count.go
diff --git a/go/src/infra/gae/libs/gae/filters/count/count.go b/go/src/infra/gae/libs/gae/filters/count/count.go
new file mode 100644
index 0000000000000000000000000000000000000000..a5478868128be9bcbb3d29e476ffe816e4f14bf6
--- /dev/null
+++ b/go/src/infra/gae/libs/gae/filters/count/count.go
@@ -0,0 +1,37 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Package count contains 'counter' filters for all the gae services. This
+// serves as a set of simple example filters, and also enables other filters
+// to test to see if certain underlying APIs are called when they should be
+// (e.g. for the datastore mcache filter, for example).
+package count
+
+import (
+	"sync/atomic"
+)
+
+// Entry is a success/fail pair for a single API method. It's returned
+// by the Counter interface.
+type Entry struct {
+	Successes int64
+	Errors    int64
+}
+
+// Total is a convenience function for getting the total number of calls to
+// this API. It's Successes+Errors.
+func (e Entry) Total() int64 { return e.Successes + e.Errors }
+
+func (e *Entry) up(errs ...error) error {

[nodir, 2015/09/19 05:45:50]

why do you accept a slice but you use only first element if present?

+	err := error(nil)
+	if len(errs) > 0 {
+		err = errs[0]
+	}
+	if err == nil {
+		atomic.AddInt64(&e.Successes, 1)
+	} else {
+		atomic.AddInt64(&e.Errors, 1)
+	}
+	return err
+}