Move promtail StreamLagLabels config to upper level config.Config

[ssncferreira]

What this PR does / why we need it:

On the refactor PR #5521 configuration StreamLagLabels was moved to a new wrapper config, meaning the promtail client configuration now has an additional configs parameter:

clients:                                                                                                              
   configs:                                                                                                                                                                     
     - external_labels:                                                                                                                                                         
         cluster: dev-eu-west-0                                                                                                                                                 
       url: http://145265:${LOCAL_COLLECTION_WRITER_KEY}@cortex-gw.loki-dev-008.svc.cluster.local/loki/api/v1/push

This is a breaking change as clients with the previous configuration will fail.
Therefore, the solution proposed in this PR is to keep the property stream_lag_labels in the client.Config as deprecated (and unused) for backward compatibility and move this property to the upper config.Config structure. The new structure is as follows:

clients:                                                                                                              
  - external_labels:                                                                                                                                                         
     cluster: dev-eu-west-0                                                                                                                                                 
  url: http://145265:${LOCAL_COLLECTION_WRITER_KEY}@cortex-gw.loki-dev-008.svc.cluster.local/loki/api/v1/push
stream_lag_labels:
  ...

Which issue(s) this PR fixes:

Special notes for your reviewer:

Checklist

• Documentation added
• Tests updated
• Add an entry in the CHANGELOG.md about the changes.

[cstyan]

I think I agree with @dannykopping's comment from slack, it feels like this would be good to nest under a new options or metrics_options named config section if we're not going to keep it under clients.

[dannykopping]

LGTM