Remove extra generic from with_http_client (#1307)

* Remove extra generic from with_http_client

This generic type parameter is not being used anywhere, so when you try to use `with_http_client`, it requires you to specify a dummy type that implements `HttpClient + 'static`.

So this does not work:

```rust
new_pipeline().with_http_client(Arc::new(client))
```

But these work, even though they don't necessarily match the actual type of `client`:

```rust
new_pipeline::<Arc<dyn HttpClient>>().with_http_client(Arc::new(client))

new_pipeline::<reqwest::Client>().with_http_client(Arc::new(client))
```

* Add test and CHANGELOG entry

Author: ekzhang

opentelemetry-datadog/CHANGELOG.md

@@ -9,6 +9,7 @@
 ### Fixed
 
 - Do not set an empty span as the active span when the propagator does not find a remote span.
+- Change type signature of `with_http_client()` to use the provided generic as argument.
 
 ## V0.8.0
 

opentelemetry-datadog/src/exporter/mod.rs

@@ -347,11 +347,8 @@ impl DatadogPipelineBuilder {
     }
 
     /// Choose the http client used by uploader
-    pub fn with_http_client<T: HttpClient + 'static>(
-        mut self,
-        client: Arc<dyn HttpClient>,
-    ) -> Self {
-        self.client = Some(client);
+    pub fn with_http_client<T: HttpClient + 'static>(mut self, client: T) -> Self {
+        self.client = Some(Arc::new(client));
         self
     }
 
@@ -496,4 +493,25 @@ mod tests {
         );
         assert!(invalid.is_err())
     }
+
+    #[derive(Debug)]
+    struct DummyClient;
+
+    #[async_trait::async_trait]
+    impl HttpClient for DummyClient {
+        async fn send(
+            &self,
+            _request: Request<Vec<u8>>,
+        ) -> Result<http::Response<bytes::Bytes>, opentelemetry_http::HttpError> {
+            Ok(http::Response::new("dummy response".into()))
+        }
+    }
+
+    #[test]
+    fn test_custom_http_client() {
+        new_pipeline()
+            .with_http_client(DummyClient)
+            .build_exporter()
+            .unwrap();
+    }
 }