Introduce luci-go components version metric.

appengine/gaemiddleware/version.go

Index: appengine/gaemiddleware/version.go
diff --git a/appengine/gaemiddleware/version.go b/appengine/gaemiddleware/version.go
new file mode 100644
index 0000000000000000000000000000000000000000..60cb611c983bdfb5900f30afadae30cbfc75a2f7
--- /dev/null
+++ b/appengine/gaemiddleware/version.go
@@ -0,0 +1,23 @@
+// Copyright 2017 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package gaemiddleware
+
+import (
+	"github.com/luci/luci-go/common/tsmon/versions"
+)
+
+// Version is a semantic version of base luci-go GAE library.
+//
+// It is bumped whenever we add new features or fix important bugs. It is
+// reported to monitoring as 'luci/components/version' string metric with
+// 'component' field set to 'github.com/luci/luci-go/appengine/gaemiddleware'.
+//
+// It allows to track what GAE apps use what version of the library, so it's
+// easier to detect stale code running in production.
+const Version = "1.0.0"
+
+func init() {
+	versions.Register("github.com/luci/luci-go/appengine/gaemiddleware", Version)
+}