feat: add e2e workspace build duration metric (#21739)

Adds coderd_template_workspace_build_duration_seconds histogram that
tracks the full duration from workspace build creation to agent ready.
This captures the complete user-perceived build time including
provisioning and agent startup.

The metric is emitted when the agent reports ready/error/timeout via the
lifecycle API, ensuring each build is counted exactly once per replica.

coderd/coderd.go

@@ -42,6 +42,7 @@ import (
 	"cdr.dev/slog/v3"
 	agentproto "github.com/coder/coder/v2/agent/proto"
 	"github.com/coder/coder/v2/buildinfo"
+	"github.com/coder/coder/v2/coderd/agentapi"
 	"github.com/coder/coder/v2/coderd/agentapi/metadatabatcher"
 	_ "github.com/coder/coder/v2/coderd/apidoc" // Used for swagger docs.
 	"github.com/coder/coder/v2/coderd/appearance"
@@ -754,6 +755,7 @@ func New(options *Options) *API {
 	api.agentProvider = stn
 	if options.DeploymentValues.Prometheus.Enable {
 		options.PrometheusRegistry.MustRegister(stn)
+		api.lifecycleMetrics = agentapi.NewLifecycleMetrics(options.PrometheusRegistry)
 	}
 	api.NetworkTelemetryBatcher = tailnet.NewNetworkTelemetryBatcher(
 		quartz.NewReal(),
@@ -1888,8 +1890,9 @@ type API struct {
 	healthCheckCache    atomic.Pointer[healthsdk.HealthcheckReport]
 	healthCheckProgress healthcheck.Progress
 
-	statsReporter   *workspacestats.Reporter
-	metadataBatcher *metadatabatcher.Batcher
+	statsReporter    *workspacestats.Reporter
+	metadataBatcher  *metadatabatcher.Batcher
+	lifecycleMetrics *agentapi.LifecycleMetrics
 
 	Acquirer *provisionerdserver.Acquirer
 	// dbRolluper rolls up template usage stats from raw agent and app