refactor frontend ELB to allow access to metric endpoint paths

[DecFox]

As part of #93, we reduced the number of load balancers and got rid of the individual service domains for our aws services. We serve the metrics for these services on the root prefixed /metric endpoint. We should make changes to allow the frontend ELB (dev.api.ooni.io) to access this path on all services and if required serve on an accessible path in the service.