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>

libs/pageserver_api/src/config.rs

@@ -134,6 +134,7 @@ pub struct ConfigToml {
     pub load_previous_heatmap: Option<bool>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub generate_unarchival_heatmap: Option<bool>,
+    pub tracing: Option<Tracing>,
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
@@ -191,6 +192,54 @@ pub enum GetVectoredConcurrentIo {
     SidecarTask,
 }
 
+#[derive(Debug, Clone, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
+pub struct Ratio {
+    pub numerator: usize,
+    pub denominator: usize,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
+pub struct OtelExporterConfig {
+    pub endpoint: String,
+    pub protocol: OtelExporterProtocol,
+    #[serde(with = "humantime_serde")]
+    pub timeout: Duration,
+}
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
+#[serde(rename_all = "kebab-case")]
+pub enum OtelExporterProtocol {
+    Grpc,
+    HttpBinary,
+    HttpJson,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
+pub struct Tracing {
+    pub sampling_ratio: Ratio,
+    pub export_config: OtelExporterConfig,
+}
+
+impl From<&OtelExporterConfig> for tracing_utils::ExportConfig {
+    fn from(val: &OtelExporterConfig) -> Self {
+        tracing_utils::ExportConfig {
+            endpoint: Some(val.endpoint.clone()),
+            protocol: val.protocol.into(),
+            timeout: val.timeout,
+        }
+    }
+}
+
+impl From<OtelExporterProtocol> for tracing_utils::Protocol {
+    fn from(val: OtelExporterProtocol) -> Self {
+        match val {
+            OtelExporterProtocol::Grpc => tracing_utils::Protocol::Grpc,
+            OtelExporterProtocol::HttpJson => tracing_utils::Protocol::HttpJson,
+            OtelExporterProtocol::HttpBinary => tracing_utils::Protocol::HttpBinary,
+        }
+    }
+}
+
 pub mod statvfs {
     pub mod mock {
         #[derive(Debug, Clone, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
@@ -537,6 +586,7 @@ impl Default for ConfigToml {
             validate_wal_contiguity: None,
             load_previous_heatmap: None,
             generate_unarchival_heatmap: None,
+            tracing: None,
         }
     }
 }