External services show with an incorrect status in the PMM inventory

Instead of creating separate tickets, as the following do relate to this issue, adding as a comment here.

• In addition to the UI giving inaccurate information to the user, it is not possible to tell the status of an external service via the API and also pmm-admin; the status field is absent. This means that is is also not possible to automatically detect if an external service is correctly configured.

• The presentation of external services in pmm-admin is different for external services and makes little sense. Where all other exporters show their own port and then the service port shows against the service, for external services the port shows against the agent and nothing shows for the service; this compounds the previous point

Before noticing that it was only the connection check that prevented me from adding an external service that required query arguments in the metric path, I wrote a simple, purpose-specific reverse proxy that could be used to point PMM to and it would then handle the request and inject the query arguments. It seems that this would solve the external service issues completely, since it could be a generic `external.go` exporter. I see little change of performance issues by routing through it, since it would be in the PMM server or on the PMM client. That would allow knowledge of status and also standardise it with the other agents.

The code, which was quickly thrown together to see if it worked: