Docs: RUST_LOG also gates span export (#135)

The subscriber's single EnvFilter fronts both the console and the OTel
layer, so RUST_LOG decides which spans exist for export. Documented in
CLAUDE.md (supported range info/debug) and at the filter in init_tracing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/otel.rs

@@ -55,6 +55,9 @@ pub fn init_tracing() {
     let tracer = tracer_provider.tracer("image-server");
     global::set_tracer_provider(tracer_provider);
 
+    // One EnvFilter in front of BOTH layers: `RUST_LOG` also decides which
+    // spans exist for export. All spans are `info`, so keep it at info/debug
+    // (see CLAUDE.md "Tracing"); `warn` turns span export off.
     tracing_subscriber::registry()
         .with(
             tracing_subscriber::EnvFilter::try_from_default_env()