Some of the specs were using namespace names instead of paths for
building URLS. This would fail since we now build a namespace with a
user's name instead of a user's username.
- In prometheus proxy api, allow variables to be replaced. For example,
if 'up{env="%{ci_environment_slug}"}' is passed to the endpoint, it
becomes 'up{env="production"}' before being sent to prometheus.
The api will proxy requests to the environment's prometheus server.
The Prometheus::ProxyService class can be reused when we add support for
group prometheus servers.
