Feature: add apmcaddy

[theodesp]

Is your feature request related to a problem? Please describe.
Caddy server is a very popular HTTPS server used in many production sites. However it has not performance retrospection out of the box.

Describe the solution you'd like
I would like to have Caddy server track APM stats and visualize in Kibana.

Describe alternatives you've considered
There is no alternative APM tracing options for Caddy at the moment.

Additional context
I have opened a relevant ticket here to track feedback. I think there is a great need to trace performance on this instance. For example here

[axw]

This is a bit tricky, I think. What would you use for transaction names? Typically the transaction name is the route pattern defined by the application, which will be unknown to Caddy or any other reverse proxy.

[theodesp]

Yes. When we add a middleware like this
httpserver.GetConfig(c).AddMiddleware(middleware)
where middleware is of type
func(next httpserver.Handler) httpserver.Handlerfunc(next httpserver.Handler) httpserver.Handler

we can only get so far.

One other option is to add a special header value similar to what Zipkin does but I think its too far fetched.

What we can do is an alternative is to use directives as metadata for example:
ServiceEndpointLabel: for labelling the service

If you think it's not worth it or it won't really work then we can close this ticket.

[axw]

I would suggest that this be implemented as an external module, primarily because it's unclear to me how it should look. Once it's proven and clear on how it would work in practice (especially with distributed tracing), then we could consider if it makes sense to bring it into the main repo. How does that sound?

[theodesp]

Cool

[axw]

@theodesp given that this isn't going to be developed in the agent in the near term, I'm closing this one.

If you do get around to implementing it at some stage, please open a topic on https://discuss.elastic.co/c/apm. The broader topic of tracing at the reverse proxy level is of relevance to the whole solution. Thanks!

[mholt]

FYI, Caddy 2 has an all-new architecture and plugin API so you might be able to do what you're wanting to now. Worth a look.

[axw]

Thanks for letting us know @mholt! Will keep it in mind, but I feel like vendor-neutral OTel instrumentation would probably be more suitable here. I see there's caddyserver/caddy#4361 already :)