chore: Comment on filtering in global subscriber

In the opentelemetry-otlp example, add a comment explaining how
filtering should prevent reentrancy into the globally installed
tracing layer from the opentelemetry stack where any dependencies
might themselves use tracing.

Author: mzabaluev

examples/opentelemetry-otlp.rs

@@ -116,6 +116,13 @@ fn init_tracing_subscriber() -> OtelGuard {
     let tracer = tracer_provider.tracer("tracing-otel-subscriber");
 
     tracing_subscriber::registry()
+        // The global level filter prevents the exporter network stack
+        // from reentering the globally installed OpenTelemetryLayer with
+        // its own spans while exporting, as the libraries should not use
+        // tracing levels below DEBUG. If the OpenTelemetry layer needs to
+        // trace spans and events with higher verbosity levels, consider using
+        // per-layer filtering to target the telemetry layer specifically,
+        // e.g. by target matching.
         .with(tracing_subscriber::filter::LevelFilter::from_level(
             Level::INFO,
         ))