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/workspaceagentsrpc.go

@@ -158,6 +158,7 @@ func (api *API) workspaceAgentRPC(rw http.ResponseWriter, r *http.Request) {
 		DerpMapUpdateFrequency:    api.Options.DERPMapUpdateFrequency,
 		ExternalAuthConfigs:       api.ExternalAuthConfigs,
 		Experiments:               api.Experiments,
+		LifecycleMetrics:          api.lifecycleMetrics,
 
 		// Optional:
 		UpdateAgentMetricsFn: api.UpdateAgentMetrics,