message: fix opentelemetry example
diff: +use opentelemetry::global;
  use std::{error::Error, thread, time::Duration};
  use tracing::{span, trace, warn};
  use tracing_attributes::instrument;
  @@ -26,6 +27,7 @@ fn main() -> Result<(), Box<dyn Error + Send + Sync + 'static>> {
  .with(opentelemetry)
  .try_init()?;
  + {
  let root = span!(tracing::Level::INFO, "app_start", work_units = 2);
  let _enter = root.enter();
  @@ -36,6 +38,9 @@ fn main() -> Result<(), Box<dyn Error + Send + Sync + 'static>> {
  warn!("About to exit!");
  trace!("status: {}", work_result);
  + }
  +
  + global::shutdown_tracer_provider();
  Ok(())
  }
  