system_kafka_consumers: tiny doc changes

docs/en/engines/table-engines/integrations/kafka.md

@@ -260,3 +260,4 @@ The number of rows in one Kafka message depends on whether the format is row-bas
 
 - [Virtual columns](../../../engines/table-engines/index.md#table_engines-virtual_columns)
 - [background_message_broker_schedule_pool_size](../../../operations/server-configuration-parameters/settings.md#background_message_broker_schedule_pool_size)
+- [system.kafka_consumers](../../../operations/system-tables/kafka-consumers.md)

docs/en/operations/system-tables/kafka_consumers.md

@@ -24,6 +24,7 @@ Columns:
 - `num_rebalance_revocations`, (UInt64) - number of times the consumer was revoked its partitions
 - `num_rebalance_assignments`, (UInt64) - number of times the consumer was assigned to Kafka cluster
 - `is_currently_used`, (UInt8) - consumer is in use
+- `rdkafka_stat` (String) - library internal statistic. See https://github.com/ClickHouse/librdkafka/blob/master/STATISTICS.md . Set `statistics_interval_ms` to disable.
 
 Example:
 

src/Storages/System/StorageSystemKafkaConsumers.cpp

@@ -46,7 +46,6 @@ NamesAndTypesList StorageSystemKafkaConsumers::getNamesAndTypes()
         {"num_rebalance_assignments", std::make_shared<DataTypeUInt64>()},
         {"is_currently_used", std::make_shared<DataTypeUInt8>()},
         {"rdkafka_stat", std::make_shared<DataTypeString>()},
-        // {"json", std::make_shared<DataTypeObject>("json", false)}}
     };
     return names_and_types;
 }