Fix deadlock in safekeeper metrics (#2566)

We had a problem where almost all of the threads were waiting on a futex syscall. More specifically:
- `/metrics` handler was inside `TimelineCollector::collect()`, waiting on a mutex for a single Timeline
- This exact timeline was inside `control_file::FileStorage::persist()`, waiting on a mutex for Lazy initialization of `PERSIST_CONTROL_FILE_SECONDS`
- `PERSIST_CONTROL_FILE_SECONDS: Lazy<Histogram>` was blocked on `prometheus::register`
- `prometheus::register` calls `DEFAULT_REGISTRY.write().register()` to take a write lock on Registry and add a new metric
- `DEFAULT_REGISTRY` lock was already taken inside `DEFAULT_REGISTRY.gather()`, which was called by `/metrics` handler to collect all metrics

This commit creates another Registry with a separate lock, to avoid deadlock in a case where `TimelineCollector` triggers registration of new metrics inside default registry.

libs/utils/src/http/endpoint.rs

@@ -9,6 +9,7 @@ use once_cell::sync::Lazy;
 use routerify::ext::RequestExt;
 use routerify::RequestInfo;
 use routerify::{Middleware, Router, RouterBuilder, RouterService};
+use tokio::task::JoinError;
 use tracing::info;
 
 use std::future::Future;
@@ -35,7 +36,13 @@ async fn prometheus_metrics_handler(_req: Request<Body>) -> Result<Response<Body
     let mut buffer = vec![];
     let encoder = TextEncoder::new();
 
-    let metrics = metrics::gather();
+    let metrics = tokio::task::spawn_blocking(move || {
+        // Currently we take a lot of mutexes while collecting metrics, so it's
+        // better to spawn a blocking task to avoid blocking the event loop.
+        metrics::gather()
+    })
+    .await
+    .map_err(|e: JoinError| ApiError::InternalServerError(e.into()))?;
     encoder.encode(&metrics, &mut buffer).unwrap();
 
     let response = Response::builder()