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/lib.rs

@@ -17,7 +17,6 @@ use opentelemetry_sdk::{
 #[cfg(feature = "tracy")]
 use tracing_tracy::TracyLayer;
 
-#[cfg(feature = "tonic")] // TODO or http
 pub mod propagate;
 
 lazy_static! {