Biased load balancing

[levkk]

Description

Add two more load balancing preferences:

• replicas_only_with_failover: Use replicas for read queries, unless they all fail, and then try the primary. Make sure to set read_write_split = "include_primary" (or leave default)
• primary_only_with_failover: Use primary for read traffic unless it's banned, and failover to replicas. Same note on read_write_split as above.

Example

[[users]]
name = "analytics"
database = "prod"
load_balancing_strategy = "replicas_only_with_failover"