chore: add metric for num in progress archive tasks (#127)

Author: wlawt

.gitignore

@@ -24,3 +24,6 @@ Thumbs.db
 # Claude
 /.claude
 /ui/.claude
+
+# e2e / load tests
+wallets.json

crates/audit/src/archiver.rs

@@ -50,6 +50,7 @@ where
                     // TODO: the integration test breaks because Minio doesn't support etag
                     let writer = self.writer.clone();
                     let metrics = self.metrics.clone();
+                    self.metrics.in_flight_archive_tasks.increment(1.0);
                     tokio::spawn(async move {
                         let archive_start = Instant::now();
                         if let Err(e) = writer.archive_event(event).await {
@@ -60,6 +61,7 @@ where
                                 .record(archive_start.elapsed().as_secs_f64());
                             metrics.events_processed.increment(1);
                         }
+                        metrics.in_flight_archive_tasks.decrement(1.0);
                     });
 
                     let commit_start = Instant::now();

crates/audit/src/metrics.rs

@@ -1,4 +1,4 @@
-use metrics::{Counter, Histogram};
+use metrics::{Counter, Gauge, Histogram};
 use metrics_derive::Metrics;
 
 #[derive(Metrics, Clone)]
@@ -33,4 +33,7 @@ pub struct Metrics {
 
     #[metric(describe = "Total S3 writes skipped due to dedup")]
     pub s3_writes_skipped: Counter,
+
+    #[metric(describe = "Number of in-flight archive tasks")]
+    pub in_flight_archive_tasks: Gauge,
 }