feat(db): exposed postgres connection pooling config

[shettyh]

Summary

As of now there is no straight forward way to control the number of postgres connections created. There are some workarounds like setting lua_socket_pool_size, but that has it own downsides as it effects the global upstream connection pool also.

Because of this kong can bring down the postgres instance in case of high load and most of request are resulting in a cache miss. As part of this change exposed config to control this behaviour.

This change does not impact the default behaviour of kong

Full changelog

• Added config pg_keepalive_timeout to control the postgres connection idle timeout
• Added config pg_pool_size to control the postgres connection pool size
• Added config pg_backlog to limit the open connections to postgres
• Added relevant test cases

[mayocream]

Related pgmoon PR:

• feature: added 'backlog' and 'pool_size' options while using ngx.socket. leafo/pgmoon#105

This PR is indeed needed for improving the Postgres connections.

[hbagdi]

@dndx Should we expose this level of detail? Can dynamic scaling of workers cause bad surprises?

[dndx]

@hbagdi As long as we provide reasonable defaults this should not be a bad thing to have. Most user don't need to touch it anyway.

[samugi]

@shettyh great job! Could you add a changelog entry in the Unreleased section please?

[chronolaw]

Let's merge it after 3.1 release.

[shettyh]

@samugi updated changelog, please check

[mayocream]

Hi @shettyh, the failing CI was fixed by #9949, please rebase.

[shettyh]

@mayocream rebased, one of the test is still failing with config error