refactor(phase7): Restore resource metrics in metrics.go

- Re-enabled all resource metrics (CPU, GPU, acquisition stats)
- Metrics are conditionally registered only when w.resources != nil
- Added nil check to prevent panics if resource manager not initialized

Build status: Compiles successfully

internal/worker/metrics.go

@@ -2,6 +2,7 @@ package worker
 
 import (
 	"net/http"
+	"strconv"
 	"time"
 
 	"github.com/prometheus/client_golang/prometheus"
@@ -135,8 +136,75 @@ func (w *Worker) setupMetricsExporter() {
 		return float64(w.config.MaxWorkers)
 	}))
 
-	// Note: Resource metrics temporarily disabled during migration
-	// These will be re-enabled once resource manager is integrated
+	// Resource metrics
+	if w.resources != nil {
+		reg.MustRegister(prometheus.NewGaugeFunc(prometheus.GaugeOpts{
+			Name:        "fetchml_resources_cpu_total",
+			Help:        "Total CPU tokens managed by the worker resource manager.",
+			ConstLabels: labels,
+		}, func() float64 {
+			return float64(w.resources.Snapshot().TotalCPU)
+		}))
+		reg.MustRegister(prometheus.NewGaugeFunc(prometheus.GaugeOpts{
+			Name:        "fetchml_resources_cpu_free",
+			Help:        "Free CPU tokens currently available in the worker resource manager.",
+			ConstLabels: labels,
+		}, func() float64 {
+			return float64(w.resources.Snapshot().FreeCPU)
+		}))
+		reg.MustRegister(prometheus.NewGaugeFunc(prometheus.GaugeOpts{
+			Name:        "fetchml_resources_acquire_total",
+			Help:        "Total resource acquisition attempts.",
+			ConstLabels: labels,
+		}, func() float64 {
+			return float64(w.resources.Snapshot().AcquireTotal)
+		}))
+		reg.MustRegister(prometheus.NewGaugeFunc(prometheus.GaugeOpts{
+			Name:        "fetchml_resources_acquire_wait_total",
+			Help:        "Total resource acquisitions that had to wait for resources.",
+			ConstLabels: labels,
+		}, func() float64 {
+			return float64(w.resources.Snapshot().AcquireWaitTotal)
+		}))
+		reg.MustRegister(prometheus.NewGaugeFunc(prometheus.GaugeOpts{
+			Name:        "fetchml_resources_acquire_timeout_total",
+			Help:        "Total resource acquisition attempts that timed out.",
+			ConstLabels: labels,
+		}, func() float64 {
+			return float64(w.resources.Snapshot().AcquireTimeoutTotal)
+		}))
+		reg.MustRegister(prometheus.NewGaugeFunc(prometheus.GaugeOpts{
+			Name:        "fetchml_resources_acquire_wait_seconds_total",
+			Help:        "Total seconds spent waiting for resources across all acquisitions.",
+			ConstLabels: labels,
+		}, func() float64 {
+			return w.resources.Snapshot().AcquireWaitSeconds
+		}))
+
+		snap := w.resources.Snapshot()
+		for i := range snap.GPUFree {
+			gpuLabels := prometheus.Labels{"worker_id": w.id, "gpu_index": strconv.Itoa(i)}
+			idx := i
+			reg.MustRegister(prometheus.NewGaugeFunc(prometheus.GaugeOpts{
+				Name:        "fetchml_resources_gpu_slots_total",
+				Help:        "Total GPU slots per GPU index.",
+				ConstLabels: gpuLabels,
+			}, func() float64 {
+				return float64(w.resources.Snapshot().SlotsPerGPU)
+			}))
+			reg.MustRegister(prometheus.NewGaugeFunc(prometheus.GaugeOpts{
+				Name:        "fetchml_resources_gpu_slots_free",
+				Help:        "Free GPU slots per GPU index.",
+				ConstLabels: gpuLabels,
+			}, func() float64 {
+				s := w.resources.Snapshot()
+				if idx < 0 || idx >= len(s.GPUFree) {
+					return 0
+				}
+				return float64(s.GPUFree[idx])
+			}))
+		}
+	}
 
 	mux := http.NewServeMux()
 	mux.Handle("/metrics", promhttp.HandlerFor(reg, promhttp.HandlerOpts{}))