feat: add Prometheus metrics to listener service

Add an optional Prometheus metrics HTTP endpoint to the listener for
monitoring deployment operations. Includes four metrics:

- homelab_deploy_deployments_total (counter with status/action/error_code)
- homelab_deploy_deployment_duration_seconds (histogram with action/success)
- homelab_deploy_deployment_in_progress (gauge)
- homelab_deploy_info (gauge with hostname/tier/role/version)

New CLI flags: --metrics-enabled, --metrics-addr (default :9972)
New NixOS options: metrics.enable, metrics.address, metrics.openFirewall

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/homelab-deploy/main.go

@@ -16,7 +16,7 @@ import (
 	"github.com/urfave/cli/v3"
 )
 
-const version = "0.1.7"
+const version = "0.1.8"
 
 func main() {
 	app := &cli.Command{
@@ -90,6 +90,15 @@ func listenerCommand() *cli.Command {
 				Usage: "NATS subject for host discovery requests",
 				Value: "deploy.discover",
 			},
+			&cli.BoolFlag{
+				Name:  "metrics-enabled",
+				Usage: "Enable Prometheus metrics endpoint",
+			},
+			&cli.StringFlag{
+				Name:  "metrics-addr",
+				Usage: "Address for Prometheus metrics HTTP server",
+				Value: ":9972",
+			},
 		},
 		Action: func(ctx context.Context, c *cli.Command) error {
 			tier := c.String("tier")
@@ -107,6 +116,9 @@ func listenerCommand() *cli.Command {
 				Timeout:         time.Duration(c.Int("timeout")) * time.Second,
 				DeploySubjects:  c.StringSlice("deploy-subject"),
 				DiscoverSubject: c.String("discover-subject"),
+				MetricsEnabled:  c.Bool("metrics-enabled"),
+				MetricsAddr:     c.String("metrics-addr"),
+				Version:         version,
 			}
 
 			logger := slog.New(slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{