Support additional record labels configuration

Related to https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/13860.

For the ci-runners service deployed with the new blue/green deployment method, we need a way to label the logs with a new label - deployment. It however doesn't exist with any other part of our infrastructure.

This MR makes such configuration optional. And it also make it configurable which prometheus label should be used as the source for additional td-agent labels (so open for future cases, different than what we have in ci-runners right now).