Change slog to tracing in comments (#7378)

* #7196

Author: chong-he

common/logging/src/sse_logging_components.rs

@@ -1,5 +1,4 @@
-// TODO(tracing) fix the comments below and remove reference of slog::Drain
-//! This module provides an implementation of `slog::Drain` that optionally writes to a channel if
+//! This module provides an implementation of `tracing_subscriber::layer::Layer` that optionally writes to a channel if
 //! there are subscribers to a HTTP SSE stream.
 
 use serde_json::json;