Flatten run_metrics_poll's nested match pyramid

Extract fetch_metrics_body(client, url) -> Result<String, String> to
handle the request/error_for_status/text chain with `?`, centralizing
the three warning messages. run_metrics_poll now matches once on the
result instead of nesting three match arms deep on the happy path.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TeyhRfMYjMditek6geDRoQ

src/engine/ingest.rs

@@ -106,25 +106,34 @@ pub async fn run_file(engine: EngineHandle, path: PathBuf) {
     }
 }
 
+async fn fetch_metrics_body(client: &reqwest::Client, url: &str) -> Result<String, String> {
+    let response = client
+        .get(url)
+        .send()
+        .await
+        .map_err(|err| format!("warning: failed to poll metrics endpoint {url}: {err}"))?;
+    let response = response
+        .error_for_status()
+        .map_err(|err| format!("warning: metrics endpoint {url} returned an error: {err}"))?;
+    response
+        .text()
+        .await
+        .map_err(|err| format!("warning: failed to read metrics response from {url}: {err}"))
+}
+
 pub async fn run_metrics_poll(engine: EngineHandle, url: String) {
     let client = reqwest::Client::new();
     loop {
-        match client.get(&url).send().await {
-            Ok(response) => match response.error_for_status() {
-                Ok(response) => match response.text().await {
-                    Ok(body) => {
-                        let now = Timestamp::now();
-                        for sample in parse_openmetrics(&body, now) {
-                            if engine.tx.send(EngineMessage::IngestMetricSample(sample)).await.is_err() {
-                                return;
-                            }
-                        }
+        match fetch_metrics_body(&client, &url).await {
+            Ok(body) => {
+                let now = Timestamp::now();
+                for sample in parse_openmetrics(&body, now) {
+                    if engine.tx.send(EngineMessage::IngestMetricSample(sample)).await.is_err() {
+                        return;
                     }
-                    Err(err) => eprintln!("warning: failed to read metrics response from {url}: {err}"),
-                },
-                Err(err) => eprintln!("warning: metrics endpoint {url} returned an error: {err}"),
-            },
-            Err(err) => eprintln!("warning: failed to poll metrics endpoint {url}: {err}"),
+                }
+            }
+            Err(warning) => eprintln!("{warning}"),
         }
         tokio::time::sleep(METRICS_POLL_INTERVAL).await;
     }