fix(probe): init probe metric gauges with zero value

[stehessel]

Description

Initialize gauage metrics to avoid complications due to undefined gauge values. See stackrox/rhacs-observability-resources#115. The timestamps default to 0, which translates to 1970-01-01. It is defined such that any actual timestamp will be larger than this value.

Checklist (Definition of Done)

• Unit and integration tests added
• Added test description under Test manual
• Documentation added if necessary (i.e. changes to dev setup, test execution, ...)
• CI and all relevant tests are passing
• Add the ticket number to the PR title if available, i.e. ROX-12345: ...
• Discussed security and business related topics privately. Will move any security and business related topics that arise to private communication channel.
• Add secret to app-interface Vault or Secrets Manager if necessary
• RDS changes were e2e tested manually
• Check AWS limits are reasonable for changes provisioning new resources

Test manual

TODO: Add manual testing efforts

# To run tests locally run:
make db/teardown db/setup db/migrate
make ocm/setup OCM_OFFLINE_TOKEN=<ocm-offline-token> OCM_ENV=development
make verify lint binary test test/integration

[stehessel]

/retest

[openshift-ci]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: janisz, stehessel
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