feat(metrics): add per-backend connection, error, protocol, and pool metrics with stale backend pruning

ts/proxies/smart-proxy/models/metrics-types.ts

@@ -67,6 +67,13 @@ export interface IMetrics {
     connections(): number;
   };
   
+  // Backend metrics
+  backends: {
+    byBackend(): Map<string, IBackendMetrics>;
+    protocols(): Map<string, string>;
+    topByErrors(limit?: number): Array<{ backend: string; errors: number }>;
+  };
+
   // Performance metrics
   percentiles: {
     connectionDuration(): { p50: number; p95: number; p99: number };
@@ -98,6 +105,21 @@ export interface IMetricsConfig {
   prometheusPrefix: string;        // Default: smartproxy_
 }
 
+/**
+ * Per-backend metrics
+ */
+export interface IBackendMetrics {
+  protocol: string;
+  activeConnections: number;
+  totalConnections: number;
+  connectErrors: number;
+  handshakeErrors: number;
+  requestErrors: number;
+  avgConnectTimeMs: number;
+  poolHitRate: number;
+  h2Failures: number;
+}
+
 /**
  * Internal interface for connection byte tracking
  */

ts/proxies/smart-proxy/rust-metrics-adapter.ts

@@ -1,4 +1,4 @@
-import type { IMetrics, IThroughputData, IThroughputHistoryPoint } from './models/metrics-types.js';
+import type { IMetrics, IBackendMetrics, IThroughputData, IThroughputHistoryPoint } from './models/metrics-types.js';
 import type { RustProxyBridge } from './rust-proxy-bridge.js';
 
 /**
@@ -169,6 +169,55 @@ export class RustMetricsAdapter implements IMetrics {
     },
   };
 
+  public backends = {
+    byBackend: (): Map<string, IBackendMetrics> => {
+      const result = new Map<string, IBackendMetrics>();
+      if (this.cache?.backends) {
+        for (const [key, bm] of Object.entries(this.cache.backends)) {
+          const m = bm as any;
+          const totalTimeUs = m.totalConnectTimeUs ?? 0;
+          const count = m.connectCount ?? 0;
+          const poolHits = m.poolHits ?? 0;
+          const poolMisses = m.poolMisses ?? 0;
+          const poolTotal = poolHits + poolMisses;
+          result.set(key, {
+            protocol: m.protocol ?? 'unknown',
+            activeConnections: m.activeConnections ?? 0,
+            totalConnections: m.totalConnections ?? 0,
+            connectErrors: m.connectErrors ?? 0,
+            handshakeErrors: m.handshakeErrors ?? 0,
+            requestErrors: m.requestErrors ?? 0,
+            avgConnectTimeMs: count > 0 ? (totalTimeUs / count) / 1000 : 0,
+            poolHitRate: poolTotal > 0 ? poolHits / poolTotal : 0,
+            h2Failures: m.h2Failures ?? 0,
+          });
+        }
+      }
+      return result;
+    },
+    protocols: (): Map<string, string> => {
+      const result = new Map<string, string>();
+      if (this.cache?.backends) {
+        for (const [key, bm] of Object.entries(this.cache.backends)) {
+          result.set(key, (bm as any).protocol ?? 'unknown');
+        }
+      }
+      return result;
+    },
+    topByErrors: (limit: number = 10): Array<{ backend: string; errors: number }> => {
+      const result: Array<{ backend: string; errors: number }> = [];
+      if (this.cache?.backends) {
+        for (const [key, bm] of Object.entries(this.cache.backends)) {
+          const m = bm as any;
+          const errors = (m.connectErrors ?? 0) + (m.handshakeErrors ?? 0) + (m.requestErrors ?? 0);
+          if (errors > 0) result.push({ backend: key, errors });
+        }
+      }
+      result.sort((a, b) => b.errors - a.errors);
+      return result.slice(0, limit);
+    },
+  };
+
   public percentiles = {
     connectionDuration: (): { p50: number; p95: number; p99: number } => {
       return { p50: 0, p95: 0, p99: 0 };