[breaking change] remove log_format

[samjsong]

As part of our new kubernetes solution, we are pursuing more declarative config all around. Some configuration that we currently support like log_format or add_timestamp could instead be done in a filter plugin like the record_modifier plugin.

However, in order to support both text type formats and json type formats, while also preserving _sumo_metadata, we need to change the expected format of the input. Now the log message to be sent to sumo is wrapped in the "message" key, as shown in the unit tests

This is a breaking change and will require a major release 2.0.0

[frankreno]

LGTM - with one thing we need to do. When we cut this release. We should provide examples in the Readme of how customers could use other fluentD plugins with this to render the logs in the exact same format. They may have a ton of searches/dashbaords and alerts that depend on this and fixing those can be a non-trivial task so they need a migration path that clearly documents this. @samjsong lets make sure we add that before releasing a new major release.

[samjsong]

Makes sense, thanks for the feedback @frankreno . I'll make sure to add some documentation as part of this PR, to clearly communicate the differences in the expected input.