META: External Connection Pooling and Custom Pool Integration

There are several related issues about using external connection poolers (like PgBouncer, RDS Proxy, etc.) or custom connection pools instead of Knex's built-in tarn.js pooling. This META issue tracks all of them.

The Core Problems:

1. No way to disable Knex's internal pooling - Users want to use external poolers like PgBouncer, AWS RDS Proxy, or cloud-managed connection pooling, but Knex always creates its own pool
2. Cannot pass pre-existing pools - Users want to share connection pools between Knex and other libraries (like express-mysql-session) but have no way to pass an existing pool to Knex
3. Cannot access Knex's pool - Users want to access the underlying pool instance for monitoring, sharing with other libraries, or custom management

Use Cases:

• Cloud environments: Using managed connection poolers (AWS RDS Proxy, Azure Database connection pooling, Google Cloud SQL connection pooling)
• PgBouncer: Transaction-mode pooling for better connection efficiency across multiple application instances
• Multi-app coordination: Sharing a single connection pool between Knex and other libraries (session stores, job queues, etc.)
• Stateless connections: AWS Data API and similar services where connections are handled externally
• Pool observability: Monitoring pool metrics, implementing custom health checks, debugging connection issues

Related Open Issues:

• How to disable knex connection pooling? #5159 - How to disable knex connection pooling?
• Use custom MySQL2 connection pool into Knex - getting stucked #6092 - Use custom MySQL2 connection pool into Knex - getting stucked
• Request: Way to specify pre-existing mysql connection pool #1824 - Request: Way to specify pre-existing mysql connection pool
• How to access connection pool database instances? #5720 - How to access connection pool database instances?

Related Closed Issues (for context):

• PgBouncer transaction pooling vs. Knex pooling? #3069 - PgBouncer transaction pooling vs. Knex pooling?

This meta issue tracks progress on supporting these external pooling scenarios.