fix(witness_generator): Only spawn 1 prometheus exporter per witness …

…generator (#2492)

## What ❔

Currently, when running with `--all-rounds`, witness generator will
spawn several prometheus exporter servers.
This is meaningless, as the metrics are collected by the global registry
anyways.
This PR changes the code so that we only spawn a single prometheus
exporter.

## Why ❔

Indeed

## Checklist

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- [ ] PR title corresponds to the body of PR (we generate changelog
entries from PRs).
- [ ] Tests for the changes have been added / updated.
- [ ] Documentation comments have been added / updated.
- [ ] Code has been formatted via `zk fmt` and `zk lint`.

prover/crates/bin/witness_generator/src/main.rs

@@ -180,32 +180,32 @@ async fn main() -> anyhow::Result<()> {
         }
     };
 
+    let prometheus_config = if use_push_gateway {
+        let prometheus_config = prometheus_config
+            .clone()
+            .context("prometheus config needed when use_push_gateway enabled")?;
+        PrometheusExporterConfig::push(
+            prometheus_config
+                .gateway_endpoint()
+                .context("gateway_endpoint needed when use_push_gateway enabled")?,
+            prometheus_config.push_interval(),
+        )
+    } else {
+        PrometheusExporterConfig::pull(prometheus_listener_port as u16)
+    };
+    let prometheus_task = prometheus_config.run(stop_receiver.clone());
+
     let mut tasks = Vec::new();
+    tasks.push(tokio::spawn(prometheus_task));
 
-    for (i, round) in rounds.iter().enumerate() {
+    for round in rounds {
         tracing::info!(
             "initializing the {:?} witness generator, batch size: {:?} with protocol_version: {:?}",
             round,
             opt.batch_size,
             &protocol_version
         );
 
-        let prometheus_config = if use_push_gateway {
-            let prometheus_config = prometheus_config
-                .clone()
-                .context("prometheus config needed when use_push_gateway enabled")?;
-            PrometheusExporterConfig::push(
-                prometheus_config
-                    .gateway_endpoint()
-                    .context("gateway_endpoint needed when use_push_gateway enabled")?,
-                prometheus_config.push_interval(),
-            )
-        } else {
-            // `u16` cast is safe since i is in range [0, 4)
-            PrometheusExporterConfig::pull(prometheus_listener_port + i as u16)
-        };
-        let prometheus_task = prometheus_config.run(stop_receiver.clone());
-
         let witness_generator_task = match round {
             AggregationRound::BasicCircuits => {
                 let setup_data_path = prover_config.setup_data_path.clone();
@@ -276,15 +276,14 @@ async fn main() -> anyhow::Result<()> {
             }
         };
 
-        tasks.push(tokio::spawn(prometheus_task));
         tasks.push(tokio::spawn(witness_generator_task));
 
         tracing::info!(
             "initialized {:?} witness generator in {:?}",
             round,
             started_at.elapsed()
         );
-        SERVER_METRICS.init_latency[&(*round).into()].set(started_at.elapsed());
+        SERVER_METRICS.init_latency[&round.into()].set(started_at.elapsed());
     }
 
     let (mut stop_signal_sender, mut stop_signal_receiver) = mpsc::channel(256);