T6994: Add textfile collector config option

[nvollmar]

Change summary

Adds option to configure textfile collector to pick up custom metrics.

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/T6994

Related PR(s)

How to test / Smoketest result

vyos:~$ /usr/libexec/vyos/tests/smoke/cli/test_service_monitoring_prometheu 
test_01_node_exporter (__main__.TestMonitoringPrometheus.test_01_node_exporter) ... ok
test_02_frr_exporter (__main__.TestMonitoringPrometheus.test_02_frr_exporter) ... ok
test_03_blackbox_exporter (__main__.TestMonitoringPrometheus.test_03_blackbox_exporter) ... ok
test_04_blackbox_exporter_with_config (__main__.TestMonitoringPrometheus.test_04_blackbox_exporter_with_config) ... 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]

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

[dmbaturin]

No objections but I suggested a tiny improvement.

[dmbaturin]

Do you think it may be worthwile to make this path a reusable constant?

[nvollmar]

I've defined a constant in service_monitoring_prometheus.py, if it would be reusable from there then maybe, otherwise I don't think it is worthwhile to make it reusable, as I don't see much reason for changing it.