feat(core-shared): MetricsProtocol + BindContext.metrics? field

packages/core-shared/src/instrumentation/metrics.interface.ts

@@ -1,14 +1,20 @@
+import type { MetricsProtocol } from "../di/bind-protocols";
+
 export type MetricAttributeValue = string | number | boolean;
 
 /**
  * Vendor-neutral metrics signal interface. Mirrors the pattern of ITracer / ILogger.
  * Three impls: NoopMetrics (noop), OtelMetrics (OTel API), RecordingMetrics (core-testing).
  *
+ * Extends MetricsProtocol from `core-shared/di/bind-protocols` so the type
+ * system enforces structural compatibility — narrowing IMetrics below the
+ * protocol surface causes a typecheck error.
+ *
  * gauge() limitation: uses UpDownCounter under the hood, which accumulates deltas.
  * True "set to absolute value" semantics require ObservableGauge with a callback —
  * deferred to a v2 interface when the first true-gauge use case lands.
  */
-export interface IMetrics {
+export interface IMetrics extends MetricsProtocol {
   /** Monotonic counter. Use for event counts (signups, errors, requests). */
   counter(
     name: string,