factor observability things into its own module

And also create a new error for the observability initialization.

src/observability.rs

@@ -0,0 +1,63 @@
+//! Facilities for observing runtime behavior
+#![warn(missing_docs, clippy::missing_docs_in_private_items)]
+
+use opentelemetry::KeyValue;
+use opentelemetry_sdk::Resource;
+use tracing_flame::FlameLayer;
+use tracing_subscriber::{layer::SubscriberExt, EnvFilter, Registry};
+
+use crate::{config::Config, error, utils::error::Result};
+
+/// Cleans up resources relating to observability when [`Drop`]ped
+pub(crate) struct Guard;
+
+impl Drop for Guard {
+    fn drop(&mut self) {
+        opentelemetry::global::shutdown_tracer_provider();
+    }
+}
+
+/// Initialize observability
+pub(crate) fn init(config: &Config) -> Result<Guard, error::Observability> {
+    if config.allow_jaeger {
+        opentelemetry::global::set_text_map_propagator(
+            opentelemetry_jaeger_propagator::Propagator::new(),
+        );
+        let tracer = opentelemetry_otlp::new_pipeline()
+            .tracing()
+            .with_trace_config(
+                opentelemetry_sdk::trace::config().with_resource(
+                    Resource::new(vec![KeyValue::new(
+                        "service.name",
+                        env!("CARGO_PKG_NAME"),
+                    )]),
+                ),
+            )
+            .with_exporter(opentelemetry_otlp::new_exporter().tonic())
+            .install_batch(opentelemetry_sdk::runtime::Tokio)?;
+        let telemetry = tracing_opentelemetry::layer().with_tracer(tracer);
+
+        let filter_layer = EnvFilter::try_new(&config.log)?;
+
+        let subscriber = Registry::default().with(filter_layer).with(telemetry);
+        tracing::subscriber::set_global_default(subscriber)?;
+    } else if config.tracing_flame {
+        let registry = Registry::default();
+        let (flame_layer, _guard) = FlameLayer::with_file("./tracing.folded")?;
+        let flame_layer = flame_layer.with_empty_samples(false);
+
+        let filter_layer = EnvFilter::new("trace,h2=off");
+
+        let subscriber = registry.with(filter_layer).with(flame_layer);
+        tracing::subscriber::set_global_default(subscriber)?;
+    } else {
+        let registry = Registry::default();
+        let fmt_layer = tracing_subscriber::fmt::Layer::new();
+        let filter_layer = EnvFilter::try_new(&config.log)?;
+
+        let subscriber = registry.with(filter_layer).with(fmt_layer);
+        tracing::subscriber::set_global_default(subscriber)?;
+    }
+
+    Ok(Guard)
+}