ghz-web path prefix

[JoshuaPostel]

Is your feature request related to a problem? Please describe.
This feature would make it easier to host ghz-web on a shared kubernetes development cluster.

Without a path prefix ghz-web makes requests to the cluster's root path:

my.shared.domain.com/:1 GET http://my.shared.domain.com/main.3cc72cfe.js net::ERR_ABORTED 503 (Service Unavailable)
favicon.ico:1 GET http://my.shared.domain.com/favicon.ico 503 (Service Unavailable)

Describe the solution you'd like
Add a config option to specify a path prefix...

#web.yaml
server:
  path_prefix: /my_namespace/ghz_web
  port: 3000

...such that requests are sent to my.shared.domain.com/my_namespace/ghz_web/... instead of my.shared.domain.com/...

Describe alternatives you've considered

• Once Prometheus exposition format output support #229 is implemented, prometheus + grafana would be a natural solution for monitoring ghz metrics run on kubernetes (avoiding the need to host ghz-web).
• There maybe other workarounds which could be implemented through kuberentes (using a subdomain or a node port). While I have not exhaustively tried these approaches, they would at the very least add additional complexity to hosting ghz-web.

Thank you for all your work on ghz, it has helped to ease the grpc learning curve.