[FEATURE]: Add support for lookup_srv for upstreams discovery

[danlsgiga]

The same way as the caddy http app, it would be nice to have not only the dial option but also the lookup_srv for upstream service discovery.

[pwFoo]

I like https://github.com/lucaslorentz/caddy-docker-proxy label configuration. Something similiar would be awesome

[francislavoie]

Docker labels are out of scope for this plugin.

[pwFoo]

So not label based or entire feature of auto generated config by other containers labels / env?

[francislavoie]

caddy-l4 has nothing to do with Docker. It's not in scope for this plugin.

[johnalotoski]

It would be great to see SRV support for dynamic upstreams here. Awesome that caddy supports SRV based lookups on the http side already. I'm trying to wrap my head around why so few proxy products support SRV lookups. AFAICT nginx (non-commercial) doesn't, traefik doesn't (equivalent functionality needs to use a DCS, like Consul, for service discovery), envoy doesn't even seem to yet.

Since the http app already has the SRV lookup feature (dynamic srv stanza in reverse_proxy for example), maybe the level of effort wouldn't be huge to re-use a good part of that for the tcp app?

[mholt]

@johnalotoski Yeah, this should be pretty straightforward to port over from the http app now.