pageserver: optionally export perf traces in OTEL format (#11140)

Based on https://github.com/neondatabase/neon/pull/11139

## Problem

We want to export performance traces from the pageserver in OTEL format.
End goal is to see them in Grafana.

## Summary of changes

https://github.com/neondatabase/neon/pull/11139 introduces the
infrastructure required to run the otel collector alongside the
pageserver.

### Design

Requirements:
1. We'd like to avoid implementing our own performance tracing stack if
possible and use the `tracing` crate if possible.
2. Ideally, we'd like zero overhead of a sampling rate of zero and be a
be able to change the tracing config for a tenant on the fly.
3. We should leave the current span hierarchy intact. This includes
adding perf traces without modifying existing tracing.

To satisfy (3) (and (2) in part) a separate span hierarchy is used.
`RequestContext` gains an optional `perf_span` member
that's only set when the request was chosen by sampling. All perf span
related methods added to `RequestContext` are no-ops for requests that
are not sampled.

This on its own is not enough for (3), so performance spans use a
separate tracing subscriber. The `tracing` crate doesn't have great
support for this, so there's a fair amount of boilerplate to override
the subscriber at all points of the perf span lifecycle.

### Perf Impact

[Periodic
pagebench](https://neonprod.grafana.net/d/ddqtbfykfqfi8d/e904990?orgId=1&from=2025-02-08T14:15:59.362Z&to=2025-03-10T14:15:59.362Z&timezone=utc)
shows no statistically significant regression with a sample ratio of 0.
There's an annotation on the dashboard on 2025-03-06.

### Overview of changes:
1. Clean up the `RequestContext` API a bit. Namely, get rid of the
`RequestContext::extend` API and use the builder instead.
2. Add pageserver level configs for tracing: sampling ratio, otel
endpoint, etc.
3. Introduce some perf span tracking utilities and expose them via
`RequestContext`. We add a `tracing::Span` wrapper to be used for perf
spans and a `tracing::Instrumented` equivalent for it. See doc comments
for reason.
4. Set up OTEL tracing infra according to configuration. A separate
runtime is used for the collector.
5. Add perf traces to the read path.

## Refs

- epic https://github.com/neondatabase/neon/issues/9873

---------

Co-authored-by: Christian Schwarz <christian@neon.tech>

pageserver/src/http/routes.rs

@@ -2697,11 +2697,12 @@ async fn getpage_at_lsn_handler_inner(
     let lsn: Option<Lsn> = parse_query_param(&request, "lsn")?;
 
     async {
-        let ctx = RequestContext::new(TaskKind::MgmtRequest, DownloadBehavior::Download);
-        // Enable read path debugging
         let timeline = active_timeline_of_active_tenant(&state.tenant_manager, tenant_shard_id, timeline_id).await?;
-        let ctx = RequestContextBuilder::extend(&ctx).read_path_debug(true)
-        .scope(context::Scope::new_timeline(&timeline)).build();
+        let ctx = RequestContextBuilder::new(TaskKind::MgmtRequest)
+            .download_behavior(DownloadBehavior::Download)
+            .scope(context::Scope::new_timeline(&timeline))
+            .read_path_debug(true)
+            .root();
 
         // Use last_record_lsn if no lsn is provided
         let lsn = lsn.unwrap_or_else(|| timeline.get_last_record_lsn());
@@ -3433,14 +3434,15 @@ async fn put_tenant_timeline_import_wal(
 
     check_permission(&request, Some(tenant_id))?;
 
-    let ctx = RequestContext::new(TaskKind::MgmtRequest, DownloadBehavior::Warn);
-
     let span = info_span!("import_wal", tenant_id=%tenant_id, timeline_id=%timeline_id, start_lsn=%start_lsn, end_lsn=%end_lsn);
     async move {
         let state = get_state(&request);
 
         let timeline = active_timeline_of_active_tenant(&state.tenant_manager, TenantShardId::unsharded(tenant_id), timeline_id).await?;
-        let ctx = RequestContextBuilder::extend(&ctx).scope(context::Scope::new_timeline(&timeline)).build();
+        let ctx = RequestContextBuilder::new(TaskKind::MgmtRequest)
+            .download_behavior(DownloadBehavior::Warn)
+            .scope(context::Scope::new_timeline(&timeline))
+            .root();
 
         let mut body = StreamReader::new(request.into_body().map(|res| {
             res.map_err(|error| {