feat: add SeaweedFS_upload_error_total Prometheus metric (PR #8788)

Add upload error counter labeled by HTTP status code, matching Go
commit 5fa5507. Code "0" indicates a transport error (no HTTP
response received). Incremented on replication failures in
do_replicated_request.

seaweed-volume/src/metrics.rs

@@ -135,6 +135,13 @@ lazy_static::lazy_static! {
         "In flight total upload size.",
     ).expect("metric can be created");
 
+    /// Upload error counter by HTTP status code. Code "0" = transport error (no response).
+    pub static ref UPLOAD_ERROR_COUNTER: IntCounterVec = IntCounterVec::new(
+        Opts::new("SeaweedFS_upload_error_total",
+            "Counter of upload errors by HTTP status code. Code 0 means transport error (no response received)."),
+        &["code"],
+    ).expect("metric can be created");
+
     // ---- Legacy aliases for backward compat with existing code ----
 
     /// Total number of volumes on this server (flat gauge).
@@ -243,6 +250,7 @@ pub fn register_metrics() {
             Box::new(CONCURRENT_UPLOAD_LIMIT.clone()),
             Box::new(INFLIGHT_DOWNLOAD_SIZE.clone()),
             Box::new(INFLIGHT_UPLOAD_SIZE.clone()),
+            Box::new(UPLOAD_ERROR_COUNTER.clone()),
             // Legacy metrics
             Box::new(VOLUMES_TOTAL.clone()),
             Box::new(DISK_SIZE_BYTES.clone()),

seaweed-volume/src/server/handlers.rs

@@ -557,8 +557,18 @@ async fn do_replicated_request(
         futures.push(async move {
             match req_builder.send().await {
                 Ok(r) if r.status().is_success() => Ok(()),
-                Ok(r) => Err(format!("{} returned status {}", url, r.status())),
-                Err(e) => Err(format!("{} failed: {}", url, e)),
+                Ok(r) => {
+                    crate::metrics::UPLOAD_ERROR_COUNTER
+                        .with_label_values(&[&r.status().as_u16().to_string()])
+                        .inc();
+                    Err(format!("{} returned status {}", url, r.status()))
+                }
+                Err(e) => {
+                    crate::metrics::UPLOAD_ERROR_COUNTER
+                        .with_label_values(&["0"])
+                        .inc();
+                    Err(format!("{} failed: {}", url, e))
+                }
             }
         });
     }