fix(metrics): 初始化时注册基线 counter 避免启动后渲染为空

- install_recorder 后立即以 absolute(0) 注册 MESSAGE_PROCESSING_ERRORS / AGENT_ITERATIONS / LLM_TOKENS_USED / TOOL_EXECUTION_TOTAL，确保首次 Prometheus 抓取即可见指标
- 新增测试验证初始化后 render 输出包含基线 counter

src/observability/metrics.rs

@@ -49,6 +49,7 @@ pub fn init_recorder() -> Option<PrometheusHandle> {
             let builder = PrometheusBuilder::new();
             match builder.install_recorder() {
                 Ok(handle) => {
+                    register_baseline_counters();
                     tracing::info!("Prometheus metrics recorder installed");
                     Some(handle)
                 }
@@ -61,6 +62,13 @@ pub fn init_recorder() -> Option<PrometheusHandle> {
         .clone()
 }
 
+fn register_baseline_counters() {
+    metrics::counter!(MESSAGE_PROCESSING_ERRORS).absolute(0);
+    metrics::counter!(AGENT_ITERATIONS).absolute(0);
+    metrics::counter!(LLM_TOKENS_USED).absolute(0);
+    metrics::counter!(TOOL_EXECUTION_TOTAL).absolute(0);
+}
+
 // ============================================================================
 // MetricsObserver — 桥接 ObserverEvent 到 metrics 宏
 // ============================================================================
@@ -156,3 +164,20 @@ pub fn default_observer() -> Arc<dyn Observer> {
 pub fn record_message_processing_error() {
     metrics::counter!(MESSAGE_PROCESSING_ERRORS).increment(1);
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn metrics_render_is_not_blank_right_after_init() {
+        let Some(handle) = init_recorder() else {
+            return;
+        };
+        let rendered = handle.render();
+        assert!(
+            rendered.contains(MESSAGE_PROCESSING_ERRORS),
+            "expected baseline counters in render output, got: {rendered:?}"
+        );
+    }
+}