[Bug]: The `reqwest` feature also tries to configure `reqwest::blocking::Client` and it might not be available

[dotansimha]

What happened?

I'm trying to compile a project for WASM runtime (wasm32-unknown-unknown) and it seems like opentelemetry-http (loaded by opentelemetry-zipkin) is trying to implement a trait for reqwest::blocking::Client but it does not exists for the WASM runtime.

Here is how my dependencies are configured:

opentelemetry-zipkin = { version = "0.19.0", default-features = false, features = [
  "reqwest-client",
] }

This is the relevant code:

https://github.com/open-telemetry/opentelemetry-rust/blob/main/opentelemetry-http/src/lib.rs#L81-L93

And the actual error:

error[E0433]: failed to resolve: could not find `blocking` in `reqwest`
  --> /.../opentelemetry-http-0.10.0/src/lib.rs:81:34
   |
81 |     impl HttpClient for reqwest::blocking::Client {
   |                                  ^^^^^^^^ could not find `blocking` in `reqwest`

I get opentelemetry-http-0.10.0 loaded as a dependency.

I'm not sure if impl HttpClient for reqwest::blocking::Client should be behind a separate feature flag? I noticed that opentelemetry-zipkin already have that?

API Version

0.x

SDK Version

opentelemetry-zipkin = { version = "0.19.0", default-features = false, features = [
  "reqwest-client",
] }

What Exporters are you seeing the problem on?

Zipkin

Relevant log output

error[E0433]: failed to resolve: could not find `blocking` in `reqwest`
  --> /.../opentelemetry-http-0.10.0/src/lib.rs:81:34
   |
81 |     impl HttpClient for reqwest::blocking::Client {
   |                                  ^^^^^^^^ could not find `blocking` in `reqwest`