Labeled swarm targets

[paweljw]

Trying to use the NODE_FILTER to do a staggered Swarm rollout is very annoying. Let's use Swarm node labels instead.

It should generally be

docker node update --label-add better-stack.collector=true <node-name>

and it can be scripted however the user desires, on their side. No labels = default config, go everywhere.

I'd add it to the docs once done. WDYT?

[curusarn]

Thanks!