Werk #14561: Prometheus: allow operation of special agent without querying targets endpoint

Previously, the Prometheus special agent would query the endpoint /api/v1/targets regardless of what was configured in the Prometheus rule. This was an additional burden on users, which did not specify any scrape targets and therefore do not need this endpoint. E.g., it can lead to additional timeouts. With this werk, the endpoint /api/v1/targets is only queried if necessary.

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