p2.7: prometheus /metrics

Default GET /metrics is Prometheus text; JSON stays at /metrics.json
and ?format=json. :8443 needs the metrics caller bearer; loopback
stays open. Restricted callers have no per-caller series and no
tavily:restricted gauge.

internal/digest/digest.go

@@ -154,6 +154,31 @@ func (g *Gate) Release() {
 	g.wakeLocked()
 }
 
+func (g *Gate) Snapshot() (inflight, waiters int) {
+	if g == nil {
+		return 0, 0
+	}
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	return g.inflight, len(g.waiters)
+}
+
+func (e *Engine) Stats() (calls uint64, fallback map[string]uint64) {
+	if e == nil {
+		return 0, nil
+	}
+	e.mu.Lock()
+	defer e.mu.Unlock()
+	if e.Fallback == nil {
+		return e.Calls, map[string]uint64{}
+	}
+	out := make(map[string]uint64, len(e.Fallback))
+	for k, v := range e.Fallback {
+		out[k] = v
+	}
+	return e.Calls, out
+}
+
 type Engine struct {
 	ChatURL    string
 	EmbedURL   string