feat: discovery of name of services emitting the logs

[sandeepsukhani]

What this PR does / why we need it:
otel specs recommends having a service name to identify the service emitting the logs. It would be good to adopt that for non-otel logs as well and add a single label to identify all the services to make it easier to discover the services and filter down the logs by service(s). This PR adds required changes to, by default, always have a service_name label if it does not exist.

service_name is defined using existing labels. It picks up values from labels defined in discover_service_name per-tenant config. The default list is configured to map one of the following labels to service_name: service, app, application, container, container_name, app_kubernetes_io_name, component, workload,job. If none of that is defined in the original labels in the stream then the service_name label would be set to value unknown_service.
The discovery of a service name can be disabled by setting the discover_service_name to an empty list.

Checklist

• Documentation added
• Tests updated
• CHANGELOG.md updated

[cyriltovena]

LGTM

see my comment.