feat(server): 可观测性 Phase 1 — 健康检查路由 + Prometheus 指标 + 连接池/事件积压监控

- 添加 /health/live 存活探针别名（原 /health + /health/ready 保留）
- 新增 metrics middleware：http_requests_total 计数器 + http_request_duration_seconds 直方图
- Prometheus exporter 独立端口 9090（可通过 ERP__SERVER__METRICS_PORT 覆盖）
- 后台任务每 30s 采样 DB 连接池活跃/空闲连接数（pg_stat_activity）
- 后台任务每 30s 采样 EventBus pending 事件积压数
- UUID 路径归一化避免高基数（/api/v1/users/:id/posts）

crates/erp-server/src/middleware/metrics.rs

@@ -0,0 +1,122 @@
+use axum::extract::Request;
+use axum::http::Method;
+use axum::middleware::Next;
+use axum::response::{IntoResponse, Response};
+use metrics::{counter, histogram};
+use std::time::Instant;
+
+/// HTTP 请求指标中间件。
+///
+/// 记录两个 Prometheus 指标：
+/// - `http_requests_total` — 计数器，标签: method, path, status
+/// - `http_request_duration_seconds` — 直方图，标签: method, path, status
+pub async fn metrics_middleware(req: Request, next: Next) -> Response {
+    let method = method_label(req.method());
+    let path = path_label(req.uri().path());
+
+    let start = Instant::now();
+    let resp = next.run(req).await;
+    let elapsed = start.elapsed();
+
+    let status = resp.status().as_u16().to_string();
+
+    let labels = [
+        ("method", method.clone()),
+        ("path", path.clone()),
+        ("status", status.clone()),
+    ];
+
+    counter!("http_requests_total", &labels).increment(1);
+    histogram!("http_request_duration_seconds", &labels).record(elapsed.as_secs_f64());
+
+    resp
+}
+
+fn method_label(method: &Method) -> String {
+    method.as_str().to_owned()
+}
+
+/// 归一化路径：将 UUID 段替换为 `:id`，避免高基数。
+fn path_label(path: &str) -> String {
+    let parts: Vec<&str> = path
+        .split('/')
+        .filter(|s| !s.is_empty())
+        .map(|s| if looks_like_uuid(s) { ":id" } else { s })
+        .collect();
+    if parts.is_empty() {
+        "/".to_string()
+    } else {
+        format!("/{}", parts.join("/"))
+    }
+}
+
+fn looks_like_uuid(s: &str) -> bool {
+    s.len() == 36
+        && s.chars().filter(|c| *c == '-').count() == 4
+        && s.chars().all(|c| c.is_ascii_hexdigit() || c == '-')
+}
+
+/// 在独立端口启动 Prometheus exporter。
+pub fn start_metrics_server(port: u16) {
+    let builder = metrics_exporter_prometheus::PrometheusBuilder::new();
+    let recorder = builder.build_recorder();
+    let handle = recorder.handle();
+
+    if let Err(e) = metrics::set_global_recorder(recorder) {
+        tracing::error!(error = %e, "Failed to install Prometheus recorder");
+        return;
+    }
+
+    tokio::spawn(async move {
+        let app = axum::Router::new()
+            .route(
+                "/metrics",
+                axum::routing::get(move || {
+                    let handle = handle.clone();
+                    async move {
+                        let body = handle.render();
+                        axum::response::IntoResponse::into_response(
+                            ([(axum::http::header::CONTENT_TYPE, "text/plain; version=0.0.4")], body),
+                        )
+                    }
+                }),
+            )
+            .fallback(|| async { axum::http::StatusCode::NOT_FOUND.into_response() as Response });
+
+        let addr = format!("0.0.0.0:{port}");
+        match tokio::net::TcpListener::bind(&addr).await {
+            Ok(listener) => {
+                tracing::info!(addr = %addr, "Prometheus metrics server listening");
+                if let Err(e) = axum::serve(listener, app).await {
+                    tracing::error!(error = %e, "Metrics server error");
+                }
+            }
+            Err(e) => {
+                tracing::error!(error = %e, addr = %addr, "Failed to bind metrics server");
+            }
+        }
+    });
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn path_label_normalizes_uuids() {
+        assert_eq!(path_label("/api/v1/users"), "/api/v1/users");
+        assert_eq!(
+            path_label("/api/v1/users/01234567-89ab-cdef-0123-456789abcdef/posts"),
+            "/api/v1/users/:id/posts"
+        );
+        assert_eq!(path_label("/"), "/");
+        assert_eq!(path_label(""), "/");
+    }
+
+    #[test]
+    fn is_uuid_checks_format() {
+        assert!(looks_like_uuid("01234567-89ab-cdef-0123-456789abcdef"));
+        assert!(!looks_like_uuid("not-a-uuid"));
+        assert!(!looks_like_uuid("short"));
+    }
+}

crates/erp-server/src/middleware/mod.rs

@@ -1,2 +1,3 @@
+pub mod metrics;
 pub mod rate_limit;
 pub mod tenant_rls;