add documentation to re-exported tonic types

Author: MathieuTricoire

opentelemetry-otlp/src/lib.rs

@@ -451,13 +451,16 @@ pub enum Protocol {
 /// Placeholder type when no exporter pipeline has been configured in telemetry pipeline.
 pub struct NoExporterConfig(());
 
+/// Re-exported types from the `tonic` crate.
 #[cfg(feature = "grpc-tonic")]
 pub mod tonic_types {
+    /// Re-exported types from `tonic::metadata`.
     pub mod metadata {
         #[doc(no_inline)]
         pub use tonic::metadata::MetadataMap;
     }
 
+    /// Re-exported types from `tonic::transport`.
     #[cfg(feature = "tls")]
     pub mod transport {
         #[doc(no_inline)]