feat(metrics): add Prometheus metrics and scrape endpoint

Add a prometheus metrics module with counters for requests, DDoS/scanner/
rate-limit decisions, active connections gauge, and request duration
histogram. Spawn a lightweight HTTP server on a configurable port
(default 9090) serving /metrics and /health endpoints.

Signed-off-by: Sienna Meridian Satterwhite <sienna@sunbeam.pt>

src/config.rs

@@ -131,8 +131,13 @@ pub struct TlsFileConfig {
 #[derive(Debug, Deserialize, Clone)]
 pub struct TelemetryConfig {
     pub otlp_endpoint: String,
+    /// Port for the Prometheus metrics scrape endpoint. 0 = disabled.
+    #[serde(default = "default_metrics_port")]
+    pub metrics_port: u16,
 }
 
+fn default_metrics_port() -> u16 { 9090 }
+
 /// A path-prefix sub-route within a virtual host.
 /// Matched longest-prefix-first when multiple entries share a prefix.
 #[derive(Debug, Deserialize, Clone)]