Feature/read metrics from unix socket npipe

[ph]

Added more logic inside the metricbeat url parser to differentiate the "http+unix" and the "http+npipe" case. This should not have any impact on any current module and allow to "add" the behavior to the helper.NewHTTP without introducing any breaking changes.

[ph]

jenkins test this please.

[ph]

Had a good chat with @ycombinator yesterday and after going through the hell together we agreed the easiest solution is to actually used a combined scheme, in that case unix+http:// to correctly differentiate the "protocol" (http) from the "transport" (unix). To do this we will add new fields in the HostData that will encapsulate the kind of transport and the transport part (the unix domain socket)) and the NewHTTP function will take it into consideration to correctly generate the appropriate dialer.

[ycombinator]

Thanks @ph. This is definitely looking nicer!

After looking at the switch statement in makeDialier, I wonder if it might cleaner to define a TransportDialerMaker interface with a makeDialer() method on it. Then instead of the various Transports being represented as enumerations, they could be actual structs that implement this interface. That way each transport will know how to setup its own dialer, which seems cleaner to me. It will also let us encapsulate the TransportPath field in only those transports that need it, rather than "leaking" it into mb.HostData.

WDYT?

[ph]

I think the method vs interface a bit equivalent in such a small set of code. But if desired I can make the change.

[ycombinator]

I'm not married to having the interface. The main point was more about having structs for different transport types and each of them implementing their own MakeDialer method, plus the unix and npipe structs encapsulating the transport path within them.

[ph]

Okay, I've made the change, I just need to fix a cyclic import and things should be fine.

[ph]

@ycombinator ready for review, had to recheck everything on windows to make sure I didn't broke anything with files with custom build tags.

[ycombinator]

Left one nitpick comment. Overall LGTM!

[ph]

@fearful-symmetry

Easiest way to test this is to have metricbeat's beat module configuration with the following:
tresting notes

- module: beat
  metricsets:
    - stats
    - state
  period: 10s
  hosts: ["http+npipe://./pipe/custom"]

And have a beat with monitoring enabled to publish it to the same named pipe:

http.enabled: true
http.host: npipe:///custom

[ph]

After creating the above. I saw that the syntax are indeed a bit different and I've created #15832