Configure 'compute_ctl' to use OpenTelemetry exporter.

This allows tracing the startup actions e.g. with Jaeger
(https://www.jaegertracing.io/). We use the "tracing-opentelemetry"
crate, which turns tracing spans into OpenTelemetry spans, so you can
use the usual "#[instrument]" directives to add tracing.

I put the tracing initialization code to a separate crate,
`tracing-utils`, so that we can reuse it in other programs. We
probably want to set up tracing in the same way in all our programs.

Co-authored-by: Joonas Koivunen <joonas@neon.tech>

libs/tracing-utils/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "tracing-utils"
+version = "0.1.0"
+edition.workspace = true
+license.workspace = true
+
+[dependencies]
+opentelemetry = { workspace = true, features=["rt-tokio"] }
+opentelemetry-otlp = { workspace = true, default_features=false, features = ["http-proto", "trace", "http", "reqwest-client"] }
+opentelemetry-semantic-conventions.workspace = true
+reqwest = { workspace = true, default-features = false, features = ["rustls-tls"] }
+tokio = { workspace = true, features = ["rt", "rt-multi-thread"] }
+tracing.workspace = true
+tracing-opentelemetry.workspace = true
+tracing-subscriber.workspace = true
+workspace_hack = { version = "0.1", path = "../../workspace_hack" }

libs/tracing-utils/src/lib.rs

@@ -0,0 +1,161 @@
+//! Helper functions to set up OpenTelemetry tracing.
+//!
+//! This comes in two variants, depending on whether you have a Tokio runtime available.
+//! If you do, call `init_tracing()`. It sets up the trace processor and exporter to use
+//! the current tokio runtime. If you don't have a runtime available, or you don't want
+//! to share the runtime with the tracing tasks, call `init_tracing_without_runtime()`
+//! instead. It sets up a dedicated single-threaded Tokio runtime for the tracing tasks.
+//!
+//! Example:
+//!
+//! ```rust,no_run
+//! use tracing_subscriber::prelude::*;
+//! use tracing_opentelemetry::OpenTelemetryLayer;
+//!
+//! #[tokio::main]
+//! async fn main() {
+//!     // Set up logging to stderr
+//!     let env_filter = tracing_subscriber::EnvFilter::try_from_default_env()
+//!         .unwrap_or_else(|_| tracing_subscriber::EnvFilter::new("info"));
+//!     let fmt_layer = tracing_subscriber::fmt::layer()
+//!         .with_target(false)
+//!         .with_writer(std::io::stderr);
+//!
+//!     // Initialize OpenTelemetry. Exports tracing spans as OpenTelemetry traces
+//!     let otlp_layer = tracing_utils::init_tracing("my_application").await.map(OpenTelemetryLayer::new);
+//!
+//!     // Put it all together
+//!     tracing_subscriber::registry()
+//!         .with(env_filter)
+//!         .with(otlp_layer)
+//!         .with(fmt_layer)
+//!         .init();
+//! }
+//! ```
+
+use opentelemetry::sdk::Resource;
+use opentelemetry::KeyValue;
+use opentelemetry_otlp::WithExportConfig;
+use opentelemetry_otlp::{OTEL_EXPORTER_OTLP_ENDPOINT, OTEL_EXPORTER_OTLP_TRACES_ENDPOINT};
+
+pub use tracing_opentelemetry::OpenTelemetryLayer;
+
+/// Set up OpenTelemetry exporter, using configuration from environment variables.
+///
+/// `service_name` is set as the OpenTelemetry 'service.name' resource (see
+/// <https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/resource/semantic_conventions/README.md#service>)
+///
+/// We try to follow the conventions for the environment variables specified in
+/// <https://opentelemetry.io/docs/reference/specification/sdk-environment-variables/>
+///
+/// However, we only support a subset of those options:
+///
+/// - OTEL_SDK_DISABLED is supported. The default is "false", meaning tracing
+///   is enabled by default. Set it to "true" to disable.
+///
+/// - We use the OTLP exporter, with HTTP protocol. Most of the OTEL_EXPORTER_OTLP_*
+///   settings specified in
+///   <https://opentelemetry.io/docs/reference/specification/protocol/exporter/>
+///   are supported, as they are handled by the `opentelemetry-otlp` crate.
+///   Settings related to other exporters have no effect.
+///
+/// - Some other settings are supported by the `opentelemetry` crate.
+///
+/// If you need some other setting, please test if it works first. And perhaps
+/// add a comment in the list above to save the effort of testing for the next
+/// person.
+///
+/// This doesn't block, but is marked as 'async' to hint that this must be called in
+/// asynchronous execution context.
+pub async fn init_tracing(service_name: &str) -> Option<opentelemetry::sdk::trace::Tracer> {
+    if std::env::var("OTEL_SDK_DISABLED") == Ok("true".to_string()) {
+        return None;
+    };
+    Some(init_tracing_internal(service_name.to_string()))
+}
+
+/// Like `init_tracing`, but creates a separate tokio Runtime for the tracing
+/// tasks.
+pub fn init_tracing_without_runtime(
+    service_name: &str,
+) -> Option<opentelemetry::sdk::trace::Tracer> {
+    if std::env::var("OTEL_SDK_DISABLED") == Ok("true".to_string()) {
+        return None;
+    };
+
+    // The opentelemetry batch processor and the OTLP exporter needs a Tokio
+    // runtime. Create a dedicated runtime for them. One thread should be
+    // enough.
+    //
+    // (Alternatively, instead of batching, we could use the "simple
+    // processor", which doesn't need Tokio, and use "reqwest-blocking"
+    // feature for the OTLP exporter, which also doesn't need Tokio.  However,
+    // batching is considered best practice, and also I have the feeling that
+    // the non-Tokio codepaths in the opentelemetry crate are less used and
+    // might be more buggy, so better to stay on the well-beaten path.)
+    //
+    // We leak the runtime so that it keeps running after we exit the
+    // function.
+    let runtime = Box::leak(Box::new(
+        tokio::runtime::Builder::new_multi_thread()
+            .enable_all()
+            .thread_name("otlp runtime thread")
+            .worker_threads(1)
+            .build()
+            .unwrap(),
+    ));
+    let _guard = runtime.enter();
+
+    Some(init_tracing_internal(service_name.to_string()))
+}
+
+fn init_tracing_internal(service_name: String) -> opentelemetry::sdk::trace::Tracer {
+    // Set up exporter from the OTEL_EXPORTER_* environment variables
+    let mut exporter = opentelemetry_otlp::new_exporter().http().with_env();
+
+    // XXX opentelemetry-otlp v0.18.0 has a bug in how it uses the
+    // OTEL_EXPORTER_OTLP_ENDPOINT env variable. According to the
+    // OpenTelemetry spec at
+    // <https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/protocol/exporter.md#endpoint-urls-for-otlphttp>,
+    // the full exporter URL is formed by appending "/v1/traces" to the value
+    // of OTEL_EXPORTER_OTLP_ENDPOINT. However, opentelemetry-otlp only does
+    // that with the grpc-tonic exporter. Other exporters, like the HTTP
+    // exporter, use the URL from OTEL_EXPORTER_OTLP_ENDPOINT as is, without
+    // appending "/v1/traces".
+    //
+    // See https://github.com/open-telemetry/opentelemetry-rust/pull/950
+    //
+    // Work around that by checking OTEL_EXPORTER_OTLP_ENDPOINT, and setting
+    // the endpoint url with the "/v1/traces" path ourselves. If the bug is
+    // fixed in a later version, we can remove this code. But if we don't
+    // remember to remove this, it won't do any harm either, as the crate will
+    // just ignore the OTEL_EXPORTER_OTLP_ENDPOINT setting when the endpoint
+    // is set directly with `with_endpoint`.
+    if std::env::var(OTEL_EXPORTER_OTLP_TRACES_ENDPOINT).is_err() {
+        if let Ok(mut endpoint) = std::env::var(OTEL_EXPORTER_OTLP_ENDPOINT) {
+            if !endpoint.ends_with('/') {
+                endpoint.push('/');
+            }
+            endpoint.push_str("v1/traces");
+            exporter = exporter.with_endpoint(endpoint);
+        }
+    }
+
+    opentelemetry_otlp::new_pipeline()
+        .tracing()
+        .with_exporter(exporter)
+        .with_trace_config(
+            opentelemetry::sdk::trace::config().with_resource(Resource::new(vec![KeyValue::new(
+                opentelemetry_semantic_conventions::resource::SERVICE_NAME,
+                service_name,
+            )])),
+        )
+        .install_batch(opentelemetry::runtime::Tokio)
+        .expect("could not initialize opentelemetry exporter")
+}
+
+// Shutdown trace pipeline gracefully, so that it has a chance to send any
+// pending traces before we exit.
+pub fn shutdown_tracing() {
+    opentelemetry::global::shutdown_tracer_provider();
+}