systemd: build: fix service startup failure when using IMDSv2

PR fluent#9845 added a service dependency "sethostname.service" for Amazon Linux 2,
which uses Amazon's Instance Metadata Service (IMDS) to query and set the
hostname. This works for instances using the legacy IMDSv1, however it fails
when admins have enforced the usage of IMDSv2. This patch is a modification of
@cpandya-we comment/work on fluent#10186. Fixes fluent#10186

Signed-off-by: Charlton Stanley <charlton@charltonstanley.com>

Author: charltonstanley

init/az2-sethostname.in

@@ -1,12 +1,21 @@
 [Unit]
-Description=Set Hostname Workaround coreos/bugs#1272
+Description=Set Hostname Workaround coreos/bugs#1272 with EC2 IMDSv2 support
 Wants=network-online.target
 After=network-online.target
 
 [Service]
 Type=oneshot
 RemainAfterExit=yes
-ExecStart=/bin/sh -c "/usr/bin/hostnamectl set-hostname $(curl -s http://169.254.169.254/latest/meta-data/hostname)"
+
+ExecStartPre=/bin/sh -c 'curl -sX PUT "http://169.254.169.254/latest/api/token" \
+  -H "X-aws-ec2-metadata-token-ttl-seconds: 21600" > /run/imds_token'
+
+ExecStartPre=/bin/sh -c 'curl -s "http://169.254.169.254/latest/meta-data/hostname" \
+  -H "X-aws-ec2-metadata-token: $(cat /run/imds_token)" > /run/ec2_hostname'
+
+ExecStart=/bin/sh -c '/usr/bin/hostnamectl set-hostname "$(cat /run/ec2_hostname)"'
+
+ExecStartPost=/bin/sh -c 'rm -f /run/imds_token /run/ec2_hostname'
 
 [Install]
-WantedBy=multi-user.target
+WantedBy=multi-user.target