Replace the hardcoded FLEET_MYSQL_MAX_OPEN_CONNS=10 in the loadtest
terraform (both the root ecs.tf stack and the infra/ stack) with a
mysql_max_open_conns variable, applied to both the writer and
read-replica pools.
Also document per-tier connection pool sizing in the AWS reference
architecture: 20 for R-class instances (25k+ hosts), 10 for the smaller
T-class instances (5k/10k hosts) whose low default max_connections
(~90/135) leaves little headroom.
<!-- Add the related story/sub-task/bug number, like Resolves#123, or
remove if NA -->
**Related issue:** Resolves#44802
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **New Features**
* Added a configurable setting for MySQL connection limits in the load
testing environment.
* Connection settings now use a shared input value instead of fixed
defaults, making it easier to tune database load behavior across
container types.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
Victor Lyuboslavsky
426de4e5a8