message: opentelemetry: add more comments to example
  This patch adds a bit more context around why we are creating a smaller
  scope for the spans, and also what happens when we call
  `global::shutdown_tracer_provider()` (that comment was copied from
  the`rust-opentelemetry` repo).
diff: @@ -38,8 +38,11 @@ fn main() -> Result<(), Box<dyn Error + Send + Sync + 'static>> {
  warn!("About to exit!");
  trace!("status: {}", work_result);
  - }
  + } // Once this scope is closed, all spans inside are closed as well
  + // Shut down the current tracer provider. This will invoke the shutdown
  + // method on all span processors. span processors should export remaining
  + // spans before return.
  global::shutdown_tracer_provider();
  Ok(())
  