shippingservice: add http client request span (#610)

* shippingservice: Add http client request span

The commit adds new http client span for outgoing calls to `/getquote`
endpoint of quoteservice via the `reqwests` client middleware instrumentation.
This provides a bit of the server/client tasks separation visibility
as well as enables span pairing with the downstream quoteservice server span.
The instrumentation uses the available `reqwests-tracing` and `tracing-opentelementry` crates.

* update shippingservice docs

* update changelog

* update doc section position

* apply syntax change suggestion

Co-authored-by: Juliano Costa <julianocosta89@outlook.com>

CHANGELOG.md

@@ -144,3 +144,5 @@ significant modifications will be credited to OpenTelemetry Authors.
 ([#587](https://github.com/open-telemetry/opentelemetry-demo/pull/587))
 * Pass product's `categories` as an input for the Ad service
 ([#600](https://github.com/open-telemetry/opentelemetry-demo/pull/600))
+* Add HTTP client instrumentation to shippingservice
+([#610](https://github.com/open-telemetry/opentelemetry-demo/pull/610))

docs/services/shippingservice.md

@@ -57,7 +57,7 @@ Be mindful of async runtime, [context
 guards](https://docs.rs/opentelemetry/latest/opentelemetry/struct.ContextGuard.html),
 and inability to move and clone `spans` when replicating from these samples.
 
-### Adding gRPC/HTTP instrumentation
+### Adding gRPC instrumentation
 
 This service receives gRPC requests, which are instrumented in the middleware.
 
@@ -107,6 +107,21 @@ You may also notice the `attributes` set on the span in this example, and
 context) the [OpenTelemetry API](https://docs.rs/opentelemetry/0.17.0/opentelemetry/trace/struct.SpanRef.html)
 will work.
 
+### Adding HTTP instrumentation
+
+A child *client* span is also produced for the outoing HTTP call to
+`quoteservice` via the `reqwest` client. This span pairs up with the
+corresponding `quoteservice` *server* span. The tracing instrumentation is
+implemented in the client middleware making use of the available
+`reqwest-middleware`, `reqwest-tracing` and `tracing-opentelementry` libraries:
+
+```rust
+    let reqwest_client = reqwest::Client::new();
+    let client = ClientBuilder::new(reqwest_client)
+        .with(TracingMiddleware::<SpanBackendWithUrl>::new())
+        .build();
+```
+
 ### Add span attributes
 
 Provided you are on the same thread, or in a context passed from a