Make OTLP exporter optional via feature "telemetry-otlp"

opentelemetry-otlp add 48 (!) extra dependencies and increases the
size of the garage binary by ~11 % (with fat LTO).

src/api/Cargo.toml

@@ -55,8 +55,9 @@ url = "2.1"
 
 opentelemetry = "0.17"
 opentelemetry-prometheus = "0.10"
-opentelemetry-otlp = "0.10"
+opentelemetry-otlp = { version = "0.10", optional = true }
 prometheus = "0.13"
 
 [features]
 k2v = [ "garage_util/k2v", "garage_model/k2v" ]
+telemetry-otlp = ["opentelemetry-otlp"]

src/garage/Cargo.toml

@@ -56,7 +56,7 @@ netapp = "0.4"
 
 opentelemetry = { version = "0.17", features = [ "rt-tokio" ] }
 opentelemetry-prometheus = "0.10"
-opentelemetry-otlp = "0.10"
+opentelemetry-otlp = { version = "0.10", optional = true }
 prometheus = "0.13"
 
 [dev-dependencies]
@@ -77,6 +77,8 @@ base64 = "0.13"
 default = [ "bundled-libs" ]
 kubernetes-discovery = [ "garage_rpc/kubernetes-discovery" ]
 k2v = [ "garage_util/k2v", "garage_api/k2v" ]
+# Exporter for the OpenTelemetry Collector.
+telemetry-otlp = [ "opentelemetry-otlp", "garage_api/telemetry-otlp" ]
 
 # NOTE: bundled-libs and system-libs should be treat as mutually exclusive;
 # exactly one of them should be enabled.

src/garage/main.rs

@@ -8,6 +8,7 @@ mod admin;
 mod cli;
 mod repair;
 mod server;
+#[cfg(feature = "telemetry-otlp")]
 mod tracing_setup;
 
 use std::net::SocketAddr;

src/garage/server.rs

@@ -15,6 +15,7 @@ use garage_web::run_web_server;
 use garage_api::k2v::api_server::K2VApiServer;
 
 use crate::admin::*;
+#[cfg(feature = "telemetry-otlp")]
 use crate::tracing_setup::*;
 
 async fn wait_from(mut chan: watch::Receiver<bool>) {
@@ -36,9 +37,14 @@ pub async fn run_server(config_file: PathBuf) -> Result<(), Error> {
 	info!("Initializing Garage main data store...");
 	let garage = Garage::new(config.clone(), background)?;
 
-	info!("Initialize tracing...");
 	if let Some(export_to) = config.admin.trace_sink {
+		info!("Initialize tracing...");
+
+		#[cfg(feature = "telemetry-otlp")]
 		init_tracing(&export_to, garage.system.id)?;
+
+		#[cfg(not(feature = "telemetry-otlp"))]
+		warn!("Garage was built without OTLP exporter, admin.trace_sink is ignored.");
 	}
 
 	info!("Initialize Admin API server and metrics collector...");