Add (optional) OpenTelemetry + Xray integration (#95) (#96)

This introduces all the necessary code to be able to send traces to
AWS Xray via `tracing-opentelemetry`. It can be optionally enabled
using the `xray` feature defined on this crate.

Also update the README.md with instructions on how to enable and use
this.

.github/workflows/ci.yml

@@ -15,10 +15,14 @@ jobs:
     - uses: actions/checkout@v2
     - name: Build
       run: cargo build --verbose
+    - name: Build with OTel feature
+      run: cargo build --verbose --features otel
     - name: Run tests
       run: cargo test --verbose
+    - name: Run tests with OTel feature
+      run: cargo test --verbose --features otel
     - name: rustfmt
       uses: actions-rs/cargo@v1
       with:
         command: fmt
-        args: --all -- --check
+        args: --all -- --check