feat: add coder_build_info metric (#24365)

_Disclaimer: produced by Claude Opus 4.6_

Adds a `coder_build_info` metric which allows operators to see which
versions of Coder are currently running.

---------

Signed-off-by: Danny Kopping <danny@coder.com>

coderd/prometheusmetrics/prometheusmetrics.go

@@ -710,6 +710,24 @@ func Experiments(registerer prometheus.Registerer, active codersdk.Experiments)
 	return nil
 }
 
+// BuildInfo registers a gauge which is always set to 1, with labels
+// describing the running server version. This follows the common
+// pattern used by Prometheus itself and many Go services.
+func BuildInfo(registerer prometheus.Registerer, version, revision string) error {
+	gauge := prometheus.NewGaugeVec(prometheus.GaugeOpts{
+		Namespace: "coderd",
+		Name:      "build_info",
+		Help:      "Describes the current build/version of the Coder server. Value is always 1.",
+	}, []string{"version", "revision"})
+	if err := registerer.Register(gauge); err != nil {
+		return err
+	}
+
+	gauge.WithLabelValues(version, revision).Set(1)
+
+	return nil
+}
+
 // filterAcceptableAgentLabels handles a slightly messy situation whereby `prometheus-aggregate-agent-stats-by` can control on
 // which labels agent stats are aggregated, but for these specific metrics in this file there is no `template` label value,
 // and therefore we have to exclude it from the list of acceptable labels.

coderd/prometheusmetrics/prometheusmetrics_test.go

@@ -906,6 +906,33 @@ func TestExperimentsMetric(t *testing.T) {
 	}
 }
 
+func TestBuildInfo(t *testing.T) {
+	t.Parallel()
+
+	reg := prometheus.NewRegistry()
+	version := "v2.15.0+abc1234"
+	revision := "abc1234def5678"
+
+	require.NoError(t, prometheusmetrics.BuildInfo(reg, version, revision))
+
+	out, err := reg.Gather()
+	require.NoError(t, err)
+	require.Len(t, out, 1)
+	require.Equal(t, "coderd_build_info", out[0].GetName())
+
+	metrics := out[0].GetMetric()
+	require.Len(t, metrics, 1)
+
+	// Labels are sorted alphabetically by Prometheus.
+	labels := metrics[0].GetLabel()
+	require.Len(t, labels, 2)
+	require.Equal(t, "revision", labels[0].GetName())
+	require.Equal(t, revision, labels[0].GetValue())
+	require.Equal(t, "version", labels[1].GetName())
+	require.Equal(t, version, labels[1].GetValue())
+	require.Equal(t, float64(1), metrics[0].GetGauge().GetValue())
+}
+
 func prepareWorkspaceAndAgent(ctx context.Context, t *testing.T, client *codersdk.Client, user codersdk.CreateFirstUserResponse, workspaceNum int) agentproto.DRPCAgentClient {
 	authToken := uuid.NewString()