[filebeat] Add a configuration option for TCP/UDP network type

[aleksmaus]

Proposed commit message

Add a configuration option for TCP/UDP network type.

Description below is copied from the ticket #40407:

The TCP input would accept these values for the network type:

• tcp (default)
• tcp4
• tcp6

The UDP input would accept these values for the network type:

• udp (default)
• udp4
• udp6

Examples

filebeat.inputs:
- type: tcp
  max_message_size: 10MiB
  host: "localhost:9000"
  network: tcp4

filebeat.inputs:
- type: udp
  max_message_size: 10KiB
  host: "localhost:8080"
  network: udp4

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Related issues

• Closes [tcp/udp] Add a configuration option for network type #40407

Screenshots

[elasticmachine]

Pinging @elastic/sec-deployment-and-devices (Team:Security-Deployment and Devices)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @aleksmaus? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[pkoutsovasilis]

LGMT! ty @aleksmaus

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[leehinman]

This is style/consistenty comment. I'm ok if you leave it as is, just want to bring it up to consider.

A common theme in the beats code base for default values is to make a defaultConfig function, that returns the struct with the default value filled in. And then when Unpack is run, if the supplied config didn't have the value, you get the default. And validate can work on the real values you want to check. An example would be:

beats/filebeat/input/tcp/input.go

Lines 50 to 67 in 6d25d46

	func configure(cfg *conf.C) (stateless.Input, error) {
	config := defaultConfig()
	if err := cfg.Unpack(&config); err != nil {
	return nil, err
	}
	return newServer(config)
	}
	func defaultConfig() config {
	return config{
	Config: tcp.Config{
	Timeout: time.Minute * 5,
	MaxMessageSize: 20 * humanize.MiByte,
	},
	LineDelimiter: "\n",
	}
	}

what do you think about using that pattern here?

[aleksmaus]

Yeah, I saw this pattern, was not sure if that is consistent across the board and if nobody is going to ask why we do set the configuration option when it's was not set.
One example:
https://github.com/elastic/beats/blob/main/filebeat/inputsource/udp/server_test.go#L62

The original ticket requirement was:

"When no option is provided, the default value (udp or tcp) is used"

So, took it literally in this case: not changing the config, and using the default network type "When no option is provided". I'd probably still leave this network() function at the point when the server is created in case if the config value gets messed up on a way to the server initialization.