add optional toggle for specific prometheus metrics

[igaskin]

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• I'd be willing to implement this feature (contributing guide)

Describe the user story

As an Atlantis admin, I would like to control which metrics are exposed on the /metrics prometheus endpoint.

Describe the solution you'd like

Have an option to the repo config to turn on specific metrics. This would reduce the size of the page in /metrics and also save on cost for downstream metrics storage.

example set of metrics to expose

                  "atlantis_project_plan_execution_time",
                  "atlantis_project_apply_execution_time",
                  "atlantis_project_plan_execution_success",
                  "atlantis_project_apply_execution_success",
                  "atlantis_project_plan_execution_success"

Describe the drawbacks of your solution

May limit discoverability of new metrics when added.

Describe alternatives you've considered

Filtering for specific metrics at the ingestion level.