backend weight and backup option

[dazoot]

In nginx / haproxy i can set weight for specific backends and backup (stand by failover) options.
Would love to have this in caddy too.

[francislavoie]

We're working on a rewrite of how upstreams are configured, and having weights is planned (because it'll support using SRV lookups for configuring upstreams, which has weights). Matt recently live streamed some of his work on this, if you feel like watching to get a better grasp of the internals: https://www.youtube.com/watch?v=hj7yzXb11jU

For backup, you can already do this, essentially, by using lb_policy first and enabling health checks so that the primary upstream is marked as unhealthy, which would trigger falling back to the next upstream.

[mholt]

SRV upstreams implemented in #4470 now honor the weight (and priority) fields of SRV records, which should do what you need.

If not, feel free to reopen this issue and explain more specifically what is required. Thanks!