Logs: Update dataplane specification to have labels field instead of attributes

[ivanahuckova]

Based on the discussion where we agreed that labels is a better option for a name of field in logs data plane format than attributes. This is because it helps us with backward compatibility and makes it easier for plugin developers to create logs data source plugins that correctly visualise logs in older versions of Grafana.

Part of https://github.com/grafana/grafana/issues/77697