Cherry-pick #15349 to 7.x: Add username/password in Metricbeat autodiscover hints

[ChrsMark]

Cherry-pick of PR #15349 to 7.x branch. Original message:

This PR closes #15115

Manual Testing

1. Enable autodiscover in metricbeat.yml

metricbeat:
  autodiscover.providers:
    - type: docker
      hints.enabled: true

2. Start metricbeat with log enabled: ./metricbeat -e -d "hints.builder"
3. docker run -l co.elastic.metrics/module=nats -l co.elastic.metrics/username=user42 --name nats nats
4. Check in Metricbeat logs for : 2019-12-16T15:53:18.712+0200 DEBUG [hints.builder] hints/metrics.go:144 generated config: {"enabled":true,"hosts":null,"metricsets":["connections","routes","stats","subscriptions"],"module":"nats","period":"1m","processors":null,"ssl":null,"timeout":"3s","username":"user42"}