T973: add basic frr_exporter implementation

[rebortg]

Change Summary

Implement basic frr_exporter feature

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/T973

Related PR(s)

Component(s) name

frr_exporter

Proposed changes

add new feature frr_exporter with vrf support.

How to test

# use defaults:
set service monitoring frr-exporter

# all config
set service monitoring frr-exporter listen-address 10.10.10.1
set service monitoring frr-exporter port 9999
set service monitoring frr-exporter vrf mgmt

Smoketest result

vyos@vyos:~$ /usr/libexec/vyos/tests/smoke/cli/test_service_monitoring_frr-exporter.py
test_01_basic_config (__main__.TestMonitoringFrrExporter.test_01_basic_config) ... ok

----------------------------------------------------------------------
Ran 1 test in 5.073s

OK

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

[github-actions]

👍
No issues in PR Title / Commit Title

[github-actions]

✅ No issues found in unused-imports check.. Please refer the workflow run

[github-actions]

CI integration 👍 passed!

Details

CI logs

• CLI Smoketests (no interfaces) 👍 passed
• CLI Smoketests (interfaces only) 👍 passed
• Config tests 👍 passed
• RAID1 tests 👍 passed
• TPM tests 👍 passed