Add version package

Migrate the `version` package from `github.com/prometheus/common`
to `github.com/prometheus/client_golang` in order to break the circular
dependency.
* Make `version` a top level package because it uses `init()` to
  populate data.

Related to: https://github.com/prometheus/common/issues/58

Signed-off-by: SuperQ <superq@gmail.com>

version/version.go

@@ -0,0 +1,154 @@
+// Copyright 2016 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package version
+
+import (
+	"bytes"
+	"fmt"
+	"runtime"
+	"runtime/debug"
+	"strings"
+	"text/template"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// Build information. Populated at build-time.
+var (
+	Version   string
+	Revision  string
+	Branch    string
+	BuildUser string
+	BuildDate string
+	GoVersion = runtime.Version()
+	GoOS      = runtime.GOOS
+	GoArch    = runtime.GOARCH
+)
+
+var (
+	computedRevision string
+	computedTags     string
+)
+
+func getRevision() string {
+	if Revision != "" {
+		return Revision
+	}
+	return computedRevision
+}
+
+func getTags() string {
+	return computedTags
+}
+
+func init() {
+	computedRevision, computedTags = computeRevision()
+}
+
+func computeRevision() (string, string) {
+	var (
+		rev      = "unknown"
+		tags     = "unknown"
+		modified bool
+	)
+
+	buildInfo, ok := debug.ReadBuildInfo()
+	if !ok {
+		return rev, tags
+	}
+	for _, v := range buildInfo.Settings {
+		if v.Key == "vcs.revision" {
+			rev = v.Value
+		}
+		if v.Key == "vcs.modified" {
+			if v.Value == "true" {
+				modified = true
+			}
+		}
+		if v.Key == "-tags" {
+			tags = v.Value
+		}
+	}
+	if modified {
+		return rev + "-modified", tags
+	}
+	return rev, tags
+}
+
+// NewCollector returns a collector that exports metrics about current version
+// information.
+func NewCollector(program string) prometheus.Collector {
+	return prometheus.NewGaugeFunc(
+		prometheus.GaugeOpts{
+			Namespace: program,
+			Name:      "build_info",
+			Help: fmt.Sprintf(
+				"A metric with a constant '1' value labeled by version, revision, branch, goversion from which %s was built, and the goos and goarch for the build.",
+				program,
+			),
+			ConstLabels: prometheus.Labels{
+				"version":   Version,
+				"revision":  getRevision(),
+				"branch":    Branch,
+				"goversion": GoVersion,
+				"goos":      GoOS,
+				"goarch":    GoArch,
+				"tags":      getTags(),
+			},
+		},
+		func() float64 { return 1 },
+	)
+}
+
+// versionInfoTmpl contains the template used by Info.
+var versionInfoTmpl = `
+{{.program}}, version {{.version}} (branch: {{.branch}}, revision: {{.revision}})
+  build user:       {{.buildUser}}
+  build date:       {{.buildDate}}
+  go version:       {{.goVersion}}
+  platform:         {{.platform}}
+  tags:             {{.tags}}
+`
+
+// Print returns version information.
+func Print(program string) string {
+	m := map[string]string{
+		"program":   program,
+		"version":   Version,
+		"revision":  getRevision(),
+		"branch":    Branch,
+		"buildUser": BuildUser,
+		"buildDate": BuildDate,
+		"goVersion": GoVersion,
+		"platform":  GoOS + "/" + GoArch,
+		"tags":      getTags(),
+	}
+	t := template.Must(template.New("version").Parse(versionInfoTmpl))
+
+	var buf bytes.Buffer
+	if err := t.ExecuteTemplate(&buf, "version", m); err != nil {
+		panic(err)
+	}
+	return strings.TrimSpace(buf.String())
+}
+
+// Info returns version, branch and revision information.
+func Info() string {
+	return fmt.Sprintf("(version=%s, branch=%s, revision=%s)", Version, Branch, getRevision())
+}
+
+// BuildContext returns goVersion, platform, buildUser and buildDate information.
+func BuildContext() string {
+	return fmt.Sprintf("(go=%s, platform=%s, user=%s, date=%s, tags=%s)", GoVersion, GoOS+"/"+GoArch, BuildUser, BuildDate, getTags())
+}