[3.5] Backport expose socket options

[jmhbnz]

Currently in main it's possible to specify --socket-reuse-port and --socket-reuse-address in the config file. However, the same cannot be done for release-3.5.

An added complication is that the format for specifying these in the config file differs slightly from the command line parameter. Example below:

socket-options:
  reuse-port: true

This backport addresses the immediate bug of configuration via file for these options not being possible in release-3.5.

In future, we should also consider aligning the way the option name should be used in both the configuration file and the command flags.

Relates to #16399.

[jmhbnz]

As another follow-up we should update our sample etcd config file to make it clear how these options should be specified.

[fuweid]

LGTM

[ahrtr]

Could you also add a changelog item for 3.5? @jmhbnz

[ahrtr]

Note: the option SocketOpts was introduced in 3.5 in #12702, so no need to backport this PR to 3.4.

[jmhbnz]

Note: the option SocketOpts was introduced in 3.5 in #12702, so no need to backport this PR to 3.4.

Hmm if the feature is not in release-3.4 why would we backport a fix for the feature to that branch? That would mean we need to backport the whole feature?

[chaochn47]

I guess "no need" here means "there is no need" :)

New 3.4 release won't consider backporting this PR.

[ahrtr]

I do not get why my previous comment caused confusion:)

• *: add support for socket options #12702 introduced option SocketOpts only in 3.5 (the master branch at that time);
• This PR [3.5] Backport expose socket options #16435 backported a bug fix related to SocketOpts (Note it isn't the same PR as 12702).

[jmhbnz]

I guess "no need" here means "there is no need" :)

Whoops completely misread lol, thanks @chaochn47 🤦🏻‍♂️