Include metrics from SHOW SLAVE HOSTS

[zoonage]

Description

Create some metrics based off of the output of SHOW SLAVE HOSTS.

Example output (very old mysql):

mysql> SHOW SLAVE HOSTS;
+-----------+----------------------------+------+-------------------+-----------+
| Server_id | Host                       | Port | Rpl_recovery_rank | Master_id |
+-----------+----------------------------+------+-------------------+-----------+
| 380239978 | backup_server_1 |    0 |                 0 |         1 | 
|  11882498 | backup_server_2 |    0 |                 0 |         1 | 
+-----------+----------------------------+------+-------------------+-----------+
3 rows in set (0.00 sec)

Example output (new mysql):

mysql> SHOW SLAVE HOSTS;
+------------+-----------+------+-----------+--------------------------------------+
| Server_id  | Host      | Port | Master_id | Slave_UUID                           |
+------------+-----------+------+-----------+--------------------------------------+
|  192168010 | iconnect2 | 3306 | 192168011 | 14cb6624-7f93-11e0-b2c0-c80aa9429562 |
| 1921680101 | athena    | 3306 | 192168011 | 07af4990-f41f-11df-a566-7ac56fdaf645 |
+------------+-----------+------+-----------+--------------------------------------+

Motivation

In our use case we have a requirement to know the number of slaves that a master is currently replication to. We have a workaround assuming that the master was previously replication by using the mysql_slave_* metrics, however it would be more reliable to check the output of SHOW SLAVE HOSTS from the master.

Proposed metrics

I've left out server_uuid to keep compatability between old MySQL and new MySQL as I don't know how much more it adds compared to the server_id.

# HELP The master ID that the slave uses to identify the master
# TYPE mysql_slave_hosts_master_id gauge
mysql_slave_hosts_master_id{host="backup_server_1", server_id="380239978"} 1

# HELP The port that the slave is connected to the master on (0 means --report-port is not set)
# TYPE mysql_slave_hosts_master_id gauge
mysql_slave_hosts_port{host="backup_server_1", server_id="380239978"} 0

[SuperQ]

Seems like a good idea to me.

I might suggest a simpler metric:

# HELP mysql_slave_hosts_info Information from SHOW SLAVE HOSTS
# TYPE mysql_slave_hosts_info gauge
mysql_slave_hosts_info{server_id="380239978",host="backup_server_1",port="0",master_id="",slave_uuid=""} 1
mysql_slave_hosts_info{server_id="192168010",host="iconnect2",port="3306",master_id="192168011",slave_uuid="14cb6624-7f93-11e0-b2c0-c80aa9429562"} 1

Of course, you wouldn't get metrics exactly like above, as you would only get one style from a server. But keeping consistent labels between the two variations of SHOW SLAVE HOSTS would allow for easier comparisons.

I don't see a large value in having the port as an actual metric.

[zoonage]

That method is much nicer