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.

src/bin/server.rs

@@ -12,11 +12,27 @@ use structopt::StructOpt;
 use tokio::net::TcpListener;
 use tokio::signal;
 
+#[cfg(feature = "otel")]
+// To be able to set the XrayPropagator
+use opentelemetry::global;
+#[cfg(feature = "otel")]
+// To configure certain options such as sampling rate
+use opentelemetry::sdk::trace as sdktrace;
+#[cfg(feature = "otel")]
+// For passing along the same XrayId across services
+use opentelemetry_aws::trace::XrayPropagator;
+#[cfg(feature = "otel")]
+// To be able to pass along the XrayId across services
+#[cfg(feature = "otel")]
+// The `Ext` traits are to allow the Registry to accept the
+// OpenTelemetry-specific types (such as `OpenTelemetryLayer`)
+use tracing_subscriber::{
+    fmt, layer::SubscriberExt, util::SubscriberInitExt, util::TryInitError, EnvFilter,
+};
+
 #[tokio::main]
 pub async fn main() -> mini_redis::Result<()> {
-    // enable logging
-    // see https://docs.rs/tracing for more info
-    tracing_subscriber::fmt::try_init()?;
+    set_up_logging()?;
 
     let cli = Cli::from_args();
     let port = cli.port.as_deref().unwrap_or(DEFAULT_PORT);
@@ -35,3 +51,46 @@ struct Cli {
     #[structopt(name = "port", long = "--port")]
     port: Option<String>,
 }
+
+#[cfg(not(feature = "otel"))]
+fn set_up_logging() -> mini_redis::Result<()> {
+    // See https://docs.rs/tracing for more info
+    tracing_subscriber::fmt::try_init()
+}
+
+#[cfg(feature = "otel")]
+fn set_up_logging() -> Result<(), TryInitError> {
+    // Set the global propagator to X-Ray propagator
+    // Note: If you need to pass the x-amzn-trace-id across services in the same trace,
+    // you will need this line. However, this requires additional code not pictured here.
+    // For a full example using hyper, see:
+    // https://github.com/open-telemetry/opentelemetry-rust/blob/main/examples/aws-xray/src/server.rs#L14-L26
+    global::set_text_map_propagator(XrayPropagator::default());
+
+    let tracer = opentelemetry_otlp::new_pipeline()
+        .tracing()
+        .with_exporter(opentelemetry_otlp::new_exporter().tonic())
+        .with_trace_config(
+            sdktrace::config()
+                .with_sampler(sdktrace::Sampler::AlwaysOn)
+                // Needed in order to convert the trace IDs into an Xray-compatible format
+                .with_id_generator(sdktrace::XrayIdGenerator::default()),
+        )
+        .install_simple()
+        .expect("Unable to initialize OtlpPipeline");
+
+    // Create a tracing layer with the configured tracer
+    let opentelemetry = tracing_opentelemetry::layer().with_tracer(tracer);
+
+    // Parse an `EnvFilter` configuration from the `RUST_LOG`
+    // environment variable.
+    let filter = EnvFilter::from_default_env();
+
+    // Use the tracing subscriber `Registry`, or any other subscriber
+    // that impls `LookupSpan`
+    tracing_subscriber::registry()
+        .with(opentelemetry)
+        .with(filter)
+        .with(fmt::Layer::default())
+        .try_init()
+}