ax(mining): replace prose struct comments with usage examples in metrics.go

AX principle 2: comments show HOW with real values, not WHAT the type
signature already states. Metrics and LatencyHistogram had prose
descriptions; replaced with concrete call-site examples.

Co-Authored-By: Charon <charon@lethean.io>

pkg/mining/metrics.go

@@ -6,8 +6,8 @@ import (
 	"time"
 )
 
-// Metrics provides simple instrumentation counters for the mining package.
-// These can be exposed via Prometheus or other metrics systems in the future.
+// snapshot := mining.GetMetricsSnapshot()
+// snapshot["miners_started"].(int64)
 type Metrics struct {
 	// API metrics
 	RequestsTotal   atomic.Int64
@@ -34,7 +34,8 @@ type Metrics struct {
 	P2PConnectionsTotal atomic.Int64
 }
 
-// LatencyHistogram tracks request latencies with basic percentile support.
+// h := mining.NewLatencyHistogram(1000)
+// h.Record(42 * time.Millisecond)
 type LatencyHistogram struct {
 	mu      sync.Mutex
 	samples []time.Duration