Add integration for service discovery & kv config stores (dynamic config)

If there's some standard service discovery to connect to like Consul, it would be cool to have Telegraf connect to that and automatically start collecting data for services that Telegraf supports.

So when a new MySQL server comes on, Telegraf will automatically start collecting data from it.

Just an idea. Users could also get this by just having Telegraf part of their deploys when they create new servers.

The issue names Consul-style service discovery, key-value configuration stores, and MySQL as an example target, but identifies no files, tests, or entry points. First define the discovery and configuration behavior and the integration boundary; done would require an agreed implementation scope plus tests showing newly discovered services can be monitored.

Written by the indexing model from the issue text.