TDG 1.11.0: describe `tdg_kafka_` metrics

[xuniq]

Product: TDG 1, TDG 2
Since: 1.11.0, 2.16.0
Root document: https://www.tarantool.io/en/tdg/1.6/admin/metrics/new_page
SME: @ olegrok @ oleg-jukovec

Details

https://github.com/tarantool/tdg/blob/master/CHANGELOG.md

• Added "tdg_kafka_broker_state" metric (broker state). Its possible values are INIT (1), DOWN, CONNECT, AUTH, APIVERSION_QUERY, AUTH_HANDSHAKE, UP, UPDATE (8), or UNKNOWN (-1).
• Added "tdg_kafka_broker_source" metric (broker source). Its possible values are learned (1), configured, internal, logical (4), or unknown (-1).
• Added "tdg_kafka_cgrp_state" metric (local consumer group handler's state). Its possible values are init (1), term, query-coord, wait-coord, wait-broker, wait-broker-transport, up (7), or unknown (-1).
• Added "tdg_kafka_cgrp_join_state" metric (local consumer group handler's join state). Its possible values are init (1), wait-join, wait-metadata, wait-sync, wait-assign-call, wait-unassign-call, wait-unassign-to-complete, wait-incr-unassign-to-complete, steady (9), or unknown (-1).

This page might be useful: https://www.tarantool.io/ru/tdg/latest/administration/monitoring/monitoring-tdg/#monitoring-kafka-brokers

Эти метрики также нужно занести в доку TDG 2.16.0