URLs in responses (#3102)

APIs must return a relative URL when referring to endpoints that share the current domain.

1. Most SDKs have the concept of a base_url and can use a relative URL immediately
2. When working with multiple deployments, some customers may point at a specific geo region. By providing relative URLs we do not need to understand which endpoint the request came to before generating absolute URLs

If referencing a resource on a different domain, the full domain must be provided.

Examples

In the example, /services/10sjf30rf3 and /metrics/10sjf30rf3 are both served from the same domain, whilst /logs is on a separate domain:

{
  "id": "10sjf30rf3",
  "name": "Example service",
  "metrics_url": "/metrics/10sjf30rf3",
  "logs_url": "https://api.example.com/vendor/1234/logs/10sjf30rf3",
}