systemd: build: Add sethostname workaround for AmazonLinux2

[cosmo0920]

In AmazonLinux2, it is built on top of coreos technology. So, this race condition bug is also existing there.
In this PR, I added a workaround which is proposed in #9506 (comment).

This workaround is only effective for AmazonLinux2.

Closes #9506

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Testing
Before we can approve your change; please submit the following in a comment:

• Example configuration file for the change

[SERVICE]
    flush        1
    daemon       Off
    log_level    info
    http_server  Off
    http_listen  0.0.0.0
    http_port    2020
    storage.metrics on

[INPUT]
    name dummy
    tag  dummy.local

    # Read interval (sec) Default: 1
    interval_sec 10

[FILTER]
    Name  record_modifier
    Match *
    Record hostname ${HOSTNAME}
    Record filter_type record_modifiler

[OUTPUT]
    name  stdout
    match *

• Debug log output from testing the change

It's log for confirming on self-created AMI on AWS EC2 instance.

[ec2-user@ip-172-31-17-102 ~]$ systemctl status fluent-bit -l
● fluent-bit.service - Fluent Bit
   Loaded: loaded (/usr/lib/systemd/system/fluent-bit.service; enabled; vendor preset: disabled)
   Active: active (running) since 月 2025-01-20 08:07:20 UTC; 52s ago
     Docs: https://docs.fluentbit.io/manual/
 Main PID: 1034 (fluent-bit)
   CGroup: /system.slice/fluent-bit.service
           └─1034 /opt/fluent-bit/bin/fluent-bit -c //etc/fluent-bit/fluent-bit.conf

 1月 20 08:07:20 ip-172-31-17-102.ap-northeast-1.compute.internal fluent-bit[1034]: [2025/01/20 08:07:20] [ info] [input:dummy:dummy.0] initializing
 1月 20 08:07:20 ip-172-31-17-102.ap-northeast-1.compute.internal fluent-bit[1034]: [2025/01/20 08:07:20] [ info] [input:dummy:dummy.0] storage_strategy='memory' (memory only)
 1月 20 08:07:20 ip-172-31-17-102.ap-northeast-1.compute.internal fluent-bit[1034]: [2025/01/20 08:07:20] [ info] [sp] stream processor started
 1月 20 08:07:20 ip-172-31-17-102.ap-northeast-1.compute.internal fluent-bit[1034]: [2025/01/20 08:07:20] [ info] [output:stdout:stdout.0] worker #0 started
 1月 20 08:07:30 ip-172-31-17-102.ap-northeast-1.compute.internal fluent-bit[1034]: [0] dummy.local: [[1737360449.913292106, {}], {"message"=>"dummy", "hostname"=>"ip-172-31-17-102.ap-northeast-1.compute.internal", "filter_type"=>"record_modifiler"}]
 1月 20 08:07:40 ip-172-31-17-102.ap-northeast-1.compute.internal fluent-bit[1034]: [0] dummy.local: [[1737360459.913322822, {}], {"message"=>"dummy", "hostname"=>"ip-172-31-17-102.ap-northeast-1.compute.internal", "filter_type"=>"record_modifiler"}]
 1月 20 08:07:50 ip-172-31-17-102.ap-northeast-1.compute.internal fluent-bit[1034]: [0] dummy.local: [[1737360469.913313391, {}], {"message"=>"dummy", "hostname"=>"ip-172-31-17-102.ap-northeast-1.compute.internal", "filter_type"=>"record_modifiler"}]
 1月 20 08:08:00 ip-172-31-17-102.ap-northeast-1.compute.internal fluent-bit[1034]: [0] dummy.local: [[1737360479.913316966, {}], {"message"=>"dummy", "hostname"=>"ip-172-31-17-102.ap-northeast-1.compute.internal", "filter_type"=>"record_modifiler"}]
 1月 20 08:08:10 ip-172-31-17-102.ap-northeast-1.compute.internal fluent-bit[1034]: [0] dummy.local: [[1737360489.913314708, {}], {"message"=>"dummy", "hostname"=>"ip-172-31-17-102.ap-northeast-1.compute.internal", "filter_type"=>"record_modifiler"}]
[ec2-user@ip-172-31-17-102 ~]$ hostname
ip-172-31-17-102.ap-northeast-1.compute.internal

Plus, the packer template for creating AMI:

packer {
  required_plugins {
    amazon = {
      version = ">= 1.2.8"
      source  = "github.com/hashicorp/amazon"
    }
  }
}

source "amazon-ebs" "amazonlinux2" {
  ami_name      = "fbit-amazon-linux2-arm64"
  instance_type = "t4g.micro"
  region        = "ap-northeast-1"
  source_ami_filter {
    filters = {
      name                             = "*amzn2-ami-hvm-*"
      root-device-type                 = "ebs"
      virtualization-type              = "hvm"
      architecture                     = "arm64"
    }
    most_recent = true
    owners      = ["amazon"]
  }
  ssh_username = "ec2-user"
}

build {
  name    = "az2-fbit-packer"
  sources = [
    "source.amazon-ebs.amazonlinux2"
  ]

  provisioner "file" {
    source = "/path/to/fluent-bit/packaging/packages/amazonlinux/2/2025-01-20-15_52_36/fluent-bit-4.0.0-1.aarch64.rpm"
    destination = "/home/ec2-user/fluent-bit.rpm"
  }

  provisioner "file" {
    source = "/path/to/GitHub/aws_az2_ami/fluent-bit.conf"
    destination = "/home/ec2-user/fluent-bit.conf"
  }

  provisioner "shell" {
    inline = [
        "sudo yum install -y /home/ec2-user/fluent-bit.rpm",
        "sudo mv /home/ec2-user/fluent-bit.conf /etc/fluent-bit/fluent-bit.conf",
        "sudo systemctl enable fluent-bit"
    ]
  }
}

• Attached Valgrind output that shows no leaks or memory corruption was found

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• Run local packaging test showing all targets (including any new ones) build.
• Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• Documentation required for this feature

Backporting

• Backport to latest stable release.

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[charltonstanley]

FYI, this PR caused #10186