Support modify external_labels in `prometheus.yml` for Prometheus

[mianhk]

Feature Request

Is your feature request related to a problem? Please describe:

Describe the feature you'd like:

When we want to custom labels for the specific cluster, we should add labels in global.external_labels.

Why the featue is needed:

Describe alternatives you've considered:

• Add relabel_config of metric_relabel_configs in additional_scrape_conf. This will replace exist label at least.
• Add label of static_configs in additional_scrape_conf. This will cover the static_configs.

Teachability, Documentation, Adoption, Migration Strategy: