Add username/password in Metricbeat autodiscover hints

[ChrsMark]

Describe the enhancement:
Currently we don't have username/password in autodiscover hints. In order to set this you need to make use of raw hint. Since these are really common configuration options we should consider adding them to the list of known hints.

Describe a specific use case for the enhancement or feature:
https://discuss.elastic.co/t/metricbeat-autodiscover-hints-for-mysql/211575/3

Implementation would be similar to #14208.

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"}

[rvillablanca]

This issue is marked as good first issue but it already has a PR in progress. Is there any label to mark some issues that have PR ?

[ChrsMark]

This issue is marked as good first issue but it already has a PR in progress. Is there any label to mark some issues that have PR ?

Hey usually they should be marked with in progress label. I just added it.