Removing the master/slave terminology

[bopopescu]

For diversity reasons, it would be nice to try to avoid 'master' and 'slave' terminology in this repository which can be associated to slavery. The master-slave terminology could be problematic for people in several countries which has the history of slavery like Romania, USA and many others. Thank you for considering the proposal. Let me know if any changes in the PR are needed, I would be happy to implement them.

[satterly]

MySql is adopting "source" / "replica" to replace "master" / "slave" not "main" / "subordinate" as you have used. See https://mysqlhighavailability.com/mysql-terminology-updates/

Also as MySql have admitted this change will take years to fully implement so you changes will need to be backwards compatible as it changes the names of metrics which will break existing setups. So I suggest it should be opt-in to begin with and then opt-out in future versions.