feat(rustproxy-http): add protocol failure suppression, h3 fallback escalation, and protocol cache metrics exposure

rust/crates/rustproxy-metrics/src/collector.rs

@@ -31,6 +31,8 @@ pub struct Metrics {
     pub total_udp_sessions: u64,
     pub total_datagrams_in: u64,
     pub total_datagrams_out: u64,
+    // Protocol detection cache snapshot (populated by RustProxy from HttpProxyService)
+    pub detected_protocols: Vec<ProtocolCacheEntryMetric>,
 }
 
 /// Per-route metrics.
@@ -76,6 +78,25 @@ pub struct BackendMetrics {
     pub h2_failures: u64,
 }
 
+/// Protocol cache entry for metrics/UI display.
+/// Populated from the HTTP proxy service's protocol detection cache.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct ProtocolCacheEntryMetric {
+    pub host: String,
+    pub port: u16,
+    pub domain: Option<String>,
+    pub protocol: String,
+    pub h3_port: Option<u16>,
+    pub age_secs: u64,
+    pub h2_suppressed: bool,
+    pub h3_suppressed: bool,
+    pub h2_cooldown_remaining_secs: Option<u64>,
+    pub h3_cooldown_remaining_secs: Option<u64>,
+    pub h2_consecutive_failures: Option<u32>,
+    pub h3_consecutive_failures: Option<u32>,
+}
+
 /// Statistics snapshot.
 #[derive(Debug, Clone, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
@@ -824,6 +845,7 @@ impl MetricsCollector {
             total_udp_sessions: self.total_udp_sessions.load(Ordering::Relaxed),
             total_datagrams_in: self.total_datagrams_in.load(Ordering::Relaxed),
             total_datagrams_out: self.total_datagrams_out.load(Ordering::Relaxed),
+            detected_protocols: vec![],
         }
     }
 }