feat: add master_instance_name and instance_type to mysql module

[OscarVanL]

Relates to my feature request: #554

fix #554

[imrannayer]

@OscarVanL thx for the PR. We already have replica code where these parameters are passed. That specific code is for Master instance. Replica code is here.

https://github.com/terraform-google-modules/terraform-google-sql-db/blob/master/modules/mysql/read_replica.tf

[OscarVanL]

@OscarVanL thx for the PR. We already have replica code where these parameters are passed. That specific code is for Master instance. Replica code is here.

https://github.com/terraform-google-modules/terraform-google-sql-db/blob/master/modules/mysql/read_replica.tf

Hi @imrannayer

I think it would help for me to elaborate on the use-case I described in the feature request more so you have more context.

I want to create a CloudSQL instance that replicates from my on-prem MySQL instance that will at a later stage become the master.

To start with, I want to create some cascading replicas from my on-prem database in CloudSQL. The instance I changed (which currently can only act as the master) will begin as the "root" replica. In the diagram below it is the root replica:

Initially, there will be no master in CloudSQL, only replicas.

Eventually once I'm ready, the "root" CloudSQL instance will be promoted to the master instance and my on-premises DB will be shut down:

This can be achieved using the fields I have exposed here to point to my sql_source_representation_instance, then later removing this field and assigning the instance_type to promote that instance.

I hope this gives some clarity on why I want the resource I edited to act as a replica.

[imrannayer]

/gcbrun

[OscarVanL]

@imrannayer Hey, apologies I found a mistake in this PR. Please hold the release of 18.2.0.

I should also prevent backups being defined for the replica instance as replicas do not support backups. I will make a follow-up PR shortly.