Introduce luci-go components version metric.

common/tsmon/versions/versions.go

Index: common/tsmon/versions/versions.go
diff --git a/common/tsmon/versions/versions.go b/common/tsmon/versions/versions.go
new file mode 100644
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+++ b/common/tsmon/versions/versions.go
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+// 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 versions allows processes to report string-valued metrics with
+// versions of various libraries they link with.
+//
+// The metric is named 'luci/components/version', and it has single field
+// 'component' that defines the logical name of the component whose version is
+// reported by the metric value.
+//
+// Having such metrics allows to easily detect processes that use stale code in
+// production.
+//
+// Various go packages can register their versions during 'init' time, and all
+// registered version will be flushed to monitoring whenever 'Report' is called.
+package versions
+
+import (
+	"sync"
+
+	"github.com/luci/luci-go/common/tsmon/field"
+	"github.com/luci/luci-go/common/tsmon/metric"
+
+	"golang.org/x/net/context"
+)
+
+var registry struct {
+	m sync.RWMutex
+	r map[string]string
+}
+
+var (
+	versionMetric = metric.NewString(
+		"luci/components/version",
+		"Versions of LUCI components linked into the process.",
+		nil,
+		field.String("component"),
+	)
+)
+
+// Register tells the library to start reporting a version for given component.
+//
+// This should usually called during 'init' time.
+//
+// The component name will be used as a value of 'component' metric field, and
+// the given version will become the actual reported metric value. It is a good
+// idea to use a fully qualified go package name as 'component'.
+func Register(component, version string) {
+	registry.m.Lock()
+	defer registry.m.Unlock()
+	if registry.r == nil {
+		registry.r = make(map[string]string, 1)
+	}
+	registry.r[component] = version
+}
+
+// Report populates 'luci/components/version' metric with versions of all
+// registered components.
+func Report(c context.Context) {
+	registry.m.RLock()
+	defer registry.m.RUnlock()
+	for component := range registry.r {
+		versionMetric.Set(c, registry.r[component], component)
+	}
+}