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.

README.md

@@ -59,6 +59,31 @@ cargo run --bin mini-redis-cli set foo bar
 cargo run --bin mini-redis-cli get foo
 ```
 
+## OpenTelemetry
+
+If you are running many instances of your application (which is usually the case
+when you are developing a cloud service, for example), you need a way to get all
+of your trace data out of your host and into a centralized place. There are many
+options here, such as Prometheus, Jaeger, DataDog, Honeycomb, AWS X-Ray etc.
+
+We leverage OpenTelemetry, because it's an open standard that allows for a
+single data format to be used for all the options mentioned above (and more).
+This eliminates the risk of vendor lock-in, since you can switch between
+providers if needed.
+
+### AWS X-Ray example
+
+To enable sending traces to X-Ray, use the `otel` feature:
+```
+RUST_LOG=debug cargo run --bin mini-redis-server --features otel
+```
+
+This will switch `tracing` to use `tracing-opentelemetry`. You will need to
+have a copy of AWSOtelCollector running on the same host.
+
+For demo purposes, you can follow the setup documented at
+https://github.com/aws-observability/aws-otel-collector/blob/main/docs/developers/docker-demo.md#run-a-single-aws-otel-collector-instance-in-docker
+
 ## Supported commands
 
 `mini-redis` currently supports the following commands.