chore: add explanation for consumer.recv().now_or_never() in examples (…

…#4)

* add explaination comments

* fmt

* refine

* refine

* fix

* fix

examples/consumer.rs

@@ -67,6 +67,17 @@ async fn main() {
 
     let consumer: StreamConsumer<IamConsumerContext> = config.create_with_context(context).unwrap();
 
+    // Uncomment the following code to get the partition list and assign the partitions to the consumer.
+    //
+    // Please note that it's necessary to call `consumer.recv().now_or_never()` to refresh the OAUTHBEARER token
+    // before calling `consumer.fetch_metadata()` here.
+    //
+    // > Note that before any SASL/OAUTHBEARER broker connection can succeed the application must call rd_kafka_oauthbearer_set_token()
+    // > once – either directly or, more typically, by invoking either rd_kafka_poll(), rd_kafka_consumer_poll(), rd_kafka_queue_poll(),
+    // > etc, in order to cause retrieval of an initial token to occur.
+    // See https://docs.confluent.io/platform/current/clients/librdkafka/html/rdkafka_8h.html#a988395722598f63396d7a1bedb22adaf
+    // for details.
+    //
     // assert!(consumer.recv().now_or_never().is_none());
     // let partition_list = {
     //     let mut list = TopicPartitionList::new();