Introduce luci-go components version metric.

common/tsmon/versions/versions.go

+// 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)
+        }
+}