Update static-discovery config to supply __address__

[goller]

Required for all non-trivial PRs

• Rebased/mergable
• Tests pass
• CHANGELOG.md updated
• Sign CLA (if not already signed)

Problem

The static-discovery configuration was difficult to use:
Previously, it had a prometheus assumption of __address__ as a target:

[[static-discovery]]
  enabled = true
  id = "mystatic"
  targets = [{ "__address__" = "localhost:9100"}]
  [static.labels]
    region = "us-east-1"

Solution

The static-discovery configuration section has been simplified to look like this:

[[static-discovery]]
  enabled = true
  id = "mystatic"
  targets = ["localhost:9100"]
  [static.labels]
    region = "us-east-1"