opentelemetry: add more comments to example (#2140)

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).

Co-authored-by: Eliza Weisman <eliza@buoyant.io>

examples/examples/opentelemetry.rs

@@ -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(())