out_splunk: added otlp metadata support

[leonardo-albertovich]

This PR adds OTLP metadata support to the splunk output plugin and it improves the way in which group metadata is exposed to client code to prevent unintended meta records from being pushed to endpoints that do not expect them.

Technical notes :

In this PR the log event decoder component has been modified to prevent unsuspecting plugins from receiving meta records that hold group attributes. In order to achieve this the default value of read_groups has been changed to FLB_FALSE and its meaning reinterpreted to mean that the client code wants raw access to group metadata records rather than having the decoder transparently providing access to the information while not delivering them directly through flb_log_event_decoder_next.

What this means is that while the client code iterates the record set using flb_log_event_decoder_next two new struct flb_log_event fields named group_metadata and group_attributes will be set to point to the appropriate msgpack_object instances if the record is part of a record group or NULL otherwise.