Allow creating a consumer without group.id

Currently if a group.id is not specified we allow the use of the consumer
for fetching metadata and watermarks. Keeping this behaviour.

src/consumer/base_consumer.rs

@@ -37,6 +37,7 @@ where
 {
     client: Client<C>,
     queue: NativeQueue,
+    group_id: Option<String>,
 }
 
 impl FromClientConfig for BaseConsumer {
@@ -78,14 +79,26 @@ where
             context,
         )?;
 
-        // Redirect rdkafka's main queue to the consumer queue so that we only
-        // need to listen to the consumer queue to observe events like
-        // rebalancings and stats.
-        unsafe { rdsys::rd_kafka_poll_set_consumer(client.native_ptr()) };
-        let queue = client.consumer_queue().ok_or_else(|| {
-            KafkaError::ClientCreation("rdkafka consumer queue not available".to_string())
-        })?;
-        Ok(BaseConsumer { client, queue })
+        let group_id = config.get("group.id").map(|s| s.to_string());
+        // If a group.id is not specified, we won't redirect the main queue to the consumer queue,
+        // allowing continued use of the consumer for fetching metadata and watermarks without the
+        // need to specify a group.id
+        let queue = if group_id.is_some() {
+            // Redirect rdkafka's main queue to the consumer queue so that we only need to listen
+            // to the consumer queue to observe events like rebalancings and stats.
+            unsafe { rdsys::rd_kafka_poll_set_consumer(client.native_ptr()) };
+            client.consumer_queue().ok_or_else(|| {
+                KafkaError::ClientCreation("rdkafka consumer queue not available".to_string())
+            })?
+        } else {
+            client.main_queue()
+        };
+
+        Ok(BaseConsumer {
+            client,
+            queue,
+            group_id,
+        })
     }
 
     /// Polls the consumer for new messages.
@@ -681,15 +694,17 @@ where
 {
     fn drop(&mut self) {
         trace!("Destroying consumer: {:?}", self.client.native_ptr());
-        let err = unsafe {
-            rdsys::rd_kafka_consumer_close_queue(self.client.native_ptr(), self.queue.ptr())
-        };
-        if !err.is_null() {
-            error!("Failed to close the consumer queue on drop");
-        }
+        if self.group_id.is_some() {
+            let err = unsafe {
+                rdsys::rd_kafka_consumer_close_queue(self.client.native_ptr(), self.queue.ptr())
+            };
+            if !err.is_null() {
+                error!("Failed to close the consumer queue on drop");
+            }
 
-        while unsafe { rdsys::rd_kafka_consumer_closed(self.client.native_ptr()) } != 1 {
-            self.poll(Duration::from_millis(100));
+            while unsafe { rdsys::rd_kafka_consumer_closed(self.client.native_ptr()) } != 1 {
+                self.poll(Duration::from_millis(100));
+            }
         }
         trace!("Consumer destroyed: {:?}", self.client.native_ptr());
     }