chore: add failed archive task counter (#134)

Author: wlawt

crates/audit/src/archiver.rs

@@ -69,6 +69,7 @@ where
                         let archive_start = Instant::now();
                         if let Err(e) = writer.archive_event(event).await {
                             error!(error = %e, "Failed to write event");
+                            metrics.failed_archive_tasks.increment(1);
                         } else {
                             metrics
                                 .archive_event_duration

crates/audit/src/metrics.rs

@@ -48,4 +48,8 @@ pub struct Metrics {
     /// Number of in-flight archive tasks.
     #[metric(describe = "Number of in-flight archive tasks")]
     pub in_flight_archive_tasks: Gauge,
+
+    /// Number of failed archive tasks.
+    #[metric(describe = "Number of failed archive tasks")]
+    pub failed_archive_tasks: Counter,
 }