Create Dynamic Reverse Proxy

Problem: We are using different proxy services for scraping/crawling where we can only add 1 whitelisted access IP. But we are having different crawlers running with different IPs. Solution: All crawlers make their request via our "dynamic reverse proxy" to the service proxies. The service proxy get all requests from the same access ip (dynmaic reverse proxy). Requirements: - It should be fast (HAProxy, Ngnix, etc). It should handle thousands of requests concurrently. - All headers should be passed through the dynamic request proxy. - We should be able to easily add new upstream proxies. Optimum without downtime of the reverse proxy. Please write a describe a brief architecture of you solution. Looking forward to you propasal!