impl(gax-internal): introduce RequestRecorder (#5130)

A request recorder is a task local variable used to collect the
observability information about a request. Using a task local variable
allows us to pass *and return* information about the request through
multiple layers without having to change all the APIs.

The name is supposed to evoke "flight recorder".

Author: coryan

src/gax-internal/src/observability.rs

@@ -42,7 +42,9 @@ pub mod grpc_tracing;
 mod client_signals;
 
 #[cfg(google_cloud_unstable_tracing)]
-pub use client_signals::{ClientSignalsExt, DurationMetric, RequestStart};
+pub use client_signals::{
+    ClientRequestAttributes, ClientSignalsExt, DurationMetric, RequestRecorder, RequestStart,
+};
 
 #[cfg(google_cloud_unstable_tracing)]
 mod client_tracing;

src/gax-internal/src/observability/client_signals.rs

@@ -14,11 +14,13 @@
 
 mod client_signals_ext;
 mod duration_metric;
+mod recorder;
 mod request_start;
 mod with_client_signals;
 
 pub use client_signals_ext::ClientSignalsExt;
 pub use duration_metric::DurationMetric;
+pub use recorder::{ClientRequestAttributes, RequestRecorder};
 pub use request_start::RequestStart;
 pub use with_client_signals::WithClientSignals;