Support multiple endpoints for memcache sampler

[eaddingtonwhite]

Currently the memcache sampler in Rezolus only supports passing a single Memcache endpoint to monitor and sample
https://github.com/twitter/rezolus/blob/master/src/config/samplers.rs#L93

rezolus/src/samplers/memcache/mod.rs

Lines 43 to 57 in fadf6f8

	let endpoint = common.config.samplers().memcache().endpoint().unwrap();
	let mut addrs = endpoint.to_socket_addrs().unwrap_or_else(|_| {
	fatal!("ERROR: endpoint address is malformed: {}", endpoint);
	});
	let address = addrs.next().unwrap_or_else(|| {
	fatal!("ERROR: failed to resolve address: {}", endpoint);
	});
	let sampler = Self {
	address,
	common,
	stream: None,
	};
	if sampler.sampler_config().enabled() {
	sampler.register();
	}

It would be nice if we could pass a comma separated list of host:port memcache endpoints to Rezolus memcache sampler. This would allow you to run a single Rezolus agent to monitor multiple Memcache process or containers on a single host. The config might look something like this:

# An example config that produces percentile metrics for specific Memcached stats
# while preserving the original metric names.

[general]
listen = "0.0.0.0:4242"
fault_tolerant = false
reading_suffix = ""

[samplers]
[samplers.memcache]
enabled = true
endpoint = "localhost:11211,localhost:11212,localhost:11213,localhost:11214"

An alternative would be to run a Rezolus agent per memcache process 1:1 but this creates additional overhead for end user to configure and run on host in this way.

[brayniac]

Interesting idea. Definitely possible to support something like this, but it also raises the question of how we'd expose these on a per-instance basis. This is a somewhat larger question in terms of how we want to support "scoped" metrics. #109

Similar considerations should be made for per disk, or per core telemetry.

That said, it should be possible to do something for this sampler without solving the entire problem. I'd welcome a PR to add this functionality to the memcache sampler - I don't currently have bandwidth to work on the implementation or a real use-case for this. But it does seem useful.