feat(metrics): add build info metric on server start (#1984)

Adds a Prometheus gauge metric set to the value of 1, with labels for go version, git revision, and Boundary version. 
Assigns the Prometheus DefaultRegisterer to the Server struct as a parameter to NewServer(), to avoid "duplicate metrics collector registration" panics from test cases.

internal/cmd/base/internal/metric/build_info.go

@@ -0,0 +1,50 @@
+// Package metric provides functions to initialize a prometheus metric
+// detailing build info
+package metric
+
+import (
+	"runtime"
+
+	"github.com/hashicorp/boundary/globals"
+	"github.com/hashicorp/boundary/version"
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+const (
+	labelGoVersion       = "goversion"
+	labelGitRevision     = "revision"
+	labelBoundaryVersion = "version"
+)
+
+// buildInfoVec is a gauge metric whose value is always equal to 1 and whose
+// labels contain the current go version, git revision, and boundary version.
+var buildInfoVec = prometheus.NewGaugeVec(
+	prometheus.GaugeOpts{
+		Namespace: globals.MetricNamespace,
+		Name:      "build_info",
+		Help:      "Gauge with labels describing go version, git revision hash, and Boundary release version.",
+	},
+	[]string{labelGoVersion, labelGitRevision, labelBoundaryVersion},
+)
+
+func retrieveBuildInfoLabels() map[string]string {
+	verInfo := version.Get()
+
+	return map[string]string{
+		labelGoVersion:       runtime.Version(),
+		labelGitRevision:     verInfo.Revision,
+		labelBoundaryVersion: verInfo.Version,
+	}
+}
+
+// InitializeBuildInfo registers the boundary_build_info metric with its
+// correct labels and sets its value to 1.
+func InitializeBuildInfo(r prometheus.Registerer) {
+	if r == nil {
+		return
+	}
+
+	r.MustRegister(buildInfoVec)
+	l := prometheus.Labels(retrieveBuildInfoLabels())
+	buildInfoVec.With(l).Set(float64(1))
+}

internal/cmd/base/option.go

@@ -4,6 +4,7 @@ import (
 	"github.com/hashicorp/boundary/internal/observability/event"
 	"github.com/hashicorp/boundary/sdk/pbs/plugin"
 	wrapping "github.com/hashicorp/go-kms-wrapping/v2"
+	"github.com/prometheus/client_golang/prometheus"
 )
 
 // getOpts - iterate the inbound Options and return a struct.
@@ -40,6 +41,7 @@ type Options struct {
 	withStatusCode                 int
 	withHostPlugin                 func() (string, plugin.HostPluginServiceClient)
 	withEventGating                bool
+	withPrometheusRegisterer       prometheus.Registerer
 }
 
 func getDefaultOptions() Options {
@@ -188,3 +190,10 @@ func WithEventGating(with bool) Option {
 		o.withEventGating = with
 	}
 }
+
+// WithPrometheusRegisterer uses the provided prometheus registerer
+func WithPrometheusRegisterer(with prometheus.Registerer) Option {
+	return func(o *Options) {
+		o.withPrometheusRegisterer = with
+	}
+}

internal/cmd/base/option_test.go

@@ -4,6 +4,7 @@ import (
 	"testing"
 
 	"github.com/hashicorp/boundary/internal/observability/event"
+	"github.com/prometheus/client_golang/prometheus"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -124,4 +125,12 @@ func Test_GetOpts(t *testing.T) {
 		opts := getOpts(WithEventGating(true))
 		assert.True(opts.withEventGating)
 	})
+	t.Run("withPrometheusRegisterer", func(t *testing.T) {
+		assert := assert.New(t)
+		pmr := prometheus.NewRegistry()
+		opts := getOpts(WithPrometheusRegisterer(pmr))
+		testOpts := getDefaultOptions()
+		testOpts.withPrometheusRegisterer = pmr
+		assert.Equal(opts, testOpts)
+	})
 }

internal/cmd/base/servers.go

@@ -18,6 +18,7 @@ import (
 	"time"
 
 	"github.com/hashicorp/boundary/globals"
+	"github.com/hashicorp/boundary/internal/cmd/base/internal/metric"
 	"github.com/hashicorp/boundary/internal/cmd/base/logging"
 	"github.com/hashicorp/boundary/internal/cmd/config"
 	"github.com/hashicorp/boundary/internal/db"
@@ -133,7 +134,15 @@ type Server struct {
 	StatusGracePeriodDuration time.Duration
 }
 
-func NewServer(cmd *Command) *Server {
+// The only option used here is WithPrometheusRegisterer; all others are ignored.
+func NewServer(cmd *Command, opt ...Option) *Server {
+	// Create a new prometheus registry here to avoid "duplicate metrics collector
+	// registration" panics in tests where new servers are called consecutively.
+	// prometheus.DefaultRegisterer and prometheus.DefaultGatherer vars need to be
+	// assigned for promhttp package to work correctly.
+	opts := getOpts(opt...)
+	metric.InitializeBuildInfo(opts.withPrometheusRegisterer)
+
 	return &Server{
 		Command:              cmd,
 		InfoKeys:             make([]string, 0, 20),
@@ -142,7 +151,7 @@ func NewServer(cmd *Command) *Server {
 		ReloadFuncsLock:      new(sync.RWMutex),
 		ReloadFuncs:          make(map[string][]reloadutil.ReloadFunc),
 		StderrLock:           new(sync.Mutex),
-		PrometheusRegisterer: prometheus.DefaultRegisterer,
+		PrometheusRegisterer: opts.withPrometheusRegisterer,
 	}
 }
 

internal/cmd/commands.go

@@ -27,6 +27,7 @@ import (
 	"github.com/hashicorp/boundary/internal/cmd/commands/version"
 
 	"github.com/mitchellh/cli"
+	"github.com/prometheus/client_golang/prometheus"
 )
 
 // Commands is the mapping of all the available commands.
@@ -36,14 +37,16 @@ func initCommands(ui, serverCmdUi cli.Ui, runOpts *RunOptions) {
 	Commands = map[string]cli.CommandFactory{
 		"server": func() (cli.Command, error) {
 			return &server.Command{
-				Server:    base.NewServer(base.NewCommand(serverCmdUi)),
+				Server: base.NewServer(base.NewCommand(serverCmdUi),
+					base.WithPrometheusRegisterer(prometheus.DefaultRegisterer)),
 				SighupCh:  base.MakeSighupCh(),
 				SigUSR2Ch: MakeSigUSR2Ch(),
 			}, nil
 		},
 		"dev": func() (cli.Command, error) {
 			return &dev.Command{
-				Server:    base.NewServer(base.NewCommand(serverCmdUi)),
+				Server: base.NewServer(base.NewCommand(serverCmdUi),
+					base.WithPrometheusRegisterer(prometheus.DefaultRegisterer)),
 				SighupCh:  base.MakeSighupCh(),
 				SigUSR2Ch: MakeSigUSR2Ch(),
 			}, nil
@@ -318,7 +321,7 @@ func initCommands(ui, serverCmdUi cli.Ui, runOpts *RunOptions) {
 		},
 		"database init": func() (cli.Command, error) {
 			return &database.InitCommand{
-				Server: base.NewServer(base.NewCommand(ui)),
+				Server: base.NewServer(base.NewCommand(ui), base.WithPrometheusRegisterer(prometheus.DefaultRegisterer)),
 			}, nil
 		},
 		"database migrate": func() (cli.Command, error) {