T5152: Get default hostname for telegraf from FQDN or hostname

[sever-sever]

Change Summary

Fix for Telegraf agent hostname isn't qualified.
Try to get the hostname from FQDN and then from the hostname Used for metrics
You may have more than one machine with different domain names
r1 domain-name foo.local, hostname myhost
r2 domain-name bar.local, hostname myhost

It helps to detect from which exactly host we get metrics for InfluxDB2.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

• https://vyos.dev/T5152

Component(s) name

telegraf

Proposed changes

How to test

VyOS configuration:

set service monitoring telegraf prometheus-client
set system domain-name 'foo.local'
set system host-name 'r1'

telegraf config:

vyos@r1# cat /run/telegraf/vyos-telegraf.conf | grep host
  hostname = "r1.foo.local"
  omit_hostname = false
[edit]
vyos@r1# 

check metrics, expected host r1.foo.local

vyos@r1# curl localhost:9273/metrics | grep ntp

internal_gather_gather_time_ns{host="r1.foo.local",input="ntpq",version="1.23.1"} 3.48097686e+08
internal_gather_metrics_gathered{host="r1.foo.local",input="ntpq",version="1.23.1"} 72
ntpq_delay{host="r1.foo.local",refid="104.156.229.103",remote="ec2-34-206-168-",state_prefix="+",stratum="3",type="u"} 126.985
ntpq_delay{host="r1.foo.local",refid="128.199.243.248",remote="ec2-122-248-201",state_prefix="+",stratum="3",type="u"} 199.314
ntpq_delay{host="r1.foo.local",refid="131.188.3.221",remote="ec2-18-193-41-1",state_prefix="*",stratum="2",type="u"} 41.884

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly