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/dcache/dcache.go

@@ -217,6 +217,15 @@ func (d *DB) PutEmbed(key string, vec []float64) {
 	d.evict()
 }
 
+func (d *DB) Stats() (digestH, digestM, embedH, embedM, searchH, searchM, evictions uint64) {
+	if d == nil {
+		return 0, 0, 0, 0, 0, 0, 0
+	}
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	return d.DigestH, d.DigestM, d.EmbedH, d.EmbedM, d.SearchH, d.SearchM, d.Evictions
+}
+
 func (d *DB) Size() int64 {
 	if d == nil {
 		return 0