Unneeded connections to unavailable brokers

[emasab]

Description

What happens:
Connection is tried to brokers not reported in Metadata response, leading to timeouts that can vary depending if broker DNS entry has been removed (quicker), or DNS is available but there's no route to host (slower), or broker is reachable but port isn't bound (quicker).
Failed connection to these removed brokers can continue until client is restarted.

Conditions:
When scaling down a cluster, or just during a cluster roll, in case there aren't available connections already, the least used broker is selected and, in that case, one of the brokers removed from the cluster temporarily or permanently (client cannot know) can be used.

How to reproduce

TBW

Checklist

Please provide the following information:

• librdkafka version (release number or git tag): any
• Apache Kafka version: any
• librdkafka client configuration: any
• Operating system: any
• Provide logs (with debug=.. as necessary) from librdkafka
• Provide broker log excerpts
• Critical issue: it can be critical depending on the duration of the timeout, otherwise client recovers but it causes increased latency

[PrayagS]

@emasab Any chances this particular change will be backported to older releases of librdkafka?

I spent quite a while thinking it's a configuration issue but looking at the code and config docs, seems like this behavior is not present at all. So it would be great if older versions can also have this patch because many clients might not update to the latest minor version right away.

EDIT: Just saw the linked PR. Happy to see active development on this.

[emasab]

Closed in #4557