Adding variable and corresponding code for diverting percentage of requests which coming to a slave towards the master

[vreitech]

The pull request adding variable and corresponding code for diverting specified percentage of requests which coming into HAProxy to a slave port towards the master.

Necessity of the feature is questionable and discussionable. AFAIK there's no any direct profit to use routing of a part of reading queries to master instead of a slave.

Changes

• added haproxy_divert_to_master var into vars/main.yml

• setting haproxy_divert_to_master to 0 to avoid changes in ansible deployment result with default configuration (which disabling any diverting of connections to master instead of slave)

• added corresponding code into haproxy.cfg.j2 template

• added corresponding code into haproxy.tmpl.j2 template

[vitabaks]

Necessity of the feature is questionable and discussionable. AFAIK there's no any direct profit to use routing of a part of reading queries to master instead of a slave.

@vreitech Thank you for submitting this PR. The idea of diverting a certain percentage of read queries from HAProxy to the master node is interesting, but its practical utility seems questionable. Generally, the purpose of directing traffic to a slave is to distribute the load and avoid overburdening the master with read queries, especially in high-traffic environments.

Your implementation suggests that a portion of read queries can be intentionally rerouted to the master. This might defy the typical use case of a read replica, which is to offload read operations from the master. Could you please elaborate on specific scenarios or use cases where this would be beneficial?

Additionally, if there's a need to direct more traffic to the master, wouldn't adjusting the existing load balancing setup or adding a new endpoint like '/read-only' to point to the master be a more straightforward solution? This approach could offer more transparency and control over traffic routing without the need for a new variable or complex configuration changes.

GET /read-only: like the above endpoint, but also includes the primary.
GET /read-only-sync: like the above endpoint, but also includes the primary.

https://patroni.readthedocs.io/en/latest/rest_api.html#health-check-endpoints