I have Kafka engine table, MV and destination MT table.
Kafka table create query:
```
CREATE TABLE abc.Kafka_TestSource (...) 
ENGINE = Kafka 
SETTINGS kafka_broker_list = '...'
, format_avro_schema_registry_url = '...'
, kafka_topic_list = '...'
, kafka_group_name = 'Test_KafkaToCH'
, kafka_format = 'AvroConfluent'
, kafka_num_consumers = 1
, kafka_thread_per_consumer = 0
```

Due to Grafana consumer group stats dashboard it reads the topic with average speed ~11k records per second.
I recreate Kafka table with `kafka_num_consumers = 9` (kafka topic has 9 partitions) and it still reads with the same speed ~11k rps, **system.metrics** also shows that only 1 pool is being used:
```
SELECT
    m.metric,
    m.value
FROM system.metrics AS m
WHERE m.metric LIKE 'Background%'

Query id: 831a9285-e91b-45a4-8a1b-6b536d6860fe

┌─metric──────────────────────────────────┬─value─┐
│ BackgroundMergesAndMutationsPoolTask    │     4 │
│ BackgroundFetchesPoolTask               │     0 │
│ BackgroundCommonPoolTask                │     0 │
│ BackgroundMovePoolTask                  │     0 │
│ BackgroundSchedulePoolTask              │     0 │
│ BackgroundBufferFlushSchedulePoolTask   │     0 │
│ BackgroundDistributedSchedulePoolTask   │     0 │
│ BackgroundMessageBrokerSchedulePoolTask │     1 │
└─────────────────────────────────────────┴───────┘
```

If I set `kafka_thread_per_consumer = 1` then **BackgroundMessageBrokerSchedulePoolTask** in **system.metrics** raises up to 9 and read speed in Grafana raises up to 100 records per second:
![pic](https://github.com/sokolov-alexey/pics/blob/main/ch_kafka.PNG?raw=true)
1. `kafka_num_consumers = 1, kafka_thread_per_consumer = 0`
2. `kafka_num_consumers = 9, kafka_thread_per_consumer = 0`
3. `kafka_num_consumers = 9, kafka_thread_per_consumer = 1`

Why `kafka_num_consumers` doesn't work without `kafka_thread_per_consumer`?

ClickHouse 21.11.4.14.
formatReadableQuantity(rows), table_id.getNameForLogs(), milliseconds);
    return some_stream_is_stalled;
}
void registerStorageKafka(StorageFactory & factory)
{
    auto creator_fn = [](const StorageFactory::Arguments & args)
    {
        ASTs & engine_args = args.engine_args;
        size_t args_count = engine_args.size();
        bool has_settings = args.storage_def->settings;
        auto kafka_settings = std::make_unique<KafkaSettings>();
        auto named_collection = getExternalDataSourceConfiguration(args.engine_args, *kafka_settings, args.getLocalContext());
        if (has_settings)
        {
            kafka_settings->loadFromQuery(*args.storage_def);
        }
        
        #define CHECK_KAFKA_STORAGE_ARGUMENT(ARG_NUM, PAR_NAME, EVAL)       \
            /* One of the four required arguments is not specified */       \
            if (args_count < (ARG_NUM) && (ARG_NUM) <= 4 &&                 \
                !kafka_settings->PAR_NAME.changed)                          \
            {                                                               \
                throw Exception(                                            \
                    "Required parameter '" #PAR_NAME "' "                   \
                    "for storage Kafka not specified",                      \
                    ErrorCodes::NUMBER_OF_ARGUMENTS_DOESNT_MATCH);          \
            }                                                               \
            if (args_count >= (ARG_NUM))                                    \
            {                                                               \
                /* The same argument is given in two places */              \