feat(tvix/tracing): http trace propagation

Introduces a helper function within tvix-tracing that returns a reqwest
tracing middleware that will ingest the traceparent if otlp is enabled.

It is feature flagged in tvix-tracing so not every consumer of that
library automatically has reqwest in its dependencies.

Tested using netcat to verify that the `traceparent` header is there if
otlp is enabled and missing if otlp feature is disabled.

Change-Id: I5abccae777b725f5ff7382e3686165383c477a39
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11886
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/tracing/src/propagate/mod.rs

@@ -1,8 +1,8 @@
 #[cfg(feature = "tonic")]
 pub mod tonic;
 
-// TODO: Helper library for reqwest. We could use
-// https://github.com/TrueLayer/reqwest-middleware/tree/main/reqwest-tracing to realise this
+#[cfg(feature = "reqwest")]
+pub mod reqwest;
 
 // TODO: Helper library for axum or another http server, see
 // https://github.com/hseeberger/hello-tracing-rs/blob/main/hello-tracing-common/src/otel/http.rs

tvix/tracing/src/propagate/reqwest.rs

@@ -0,0 +1,13 @@
+use reqwest_tracing::{SpanBackendWithUrl, TracingMiddleware};
+
+/// Returns a new tracing middleware which can be used with reqwest_middleware.
+/// It will then write the `traceparent` in the header on the request and additionally records the
+/// `url` into `http.url`.
+///
+/// If otlp feature is disabled, this will not insert a `traceparent` into the header. It will
+/// basically function as a noop.
+///
+/// `traceparent` => https://www.w3.org/TR/trace-context/#trace-context-http-headers-format
+pub fn tracing_middleware() -> TracingMiddleware<SpanBackendWithUrl> {
+    TracingMiddleware::<SpanBackendWithUrl>::new()
+}