Add support for adding metadata entries as part of add_entries processor

[kkondaka]

Is your feature request related to a problem? Please describe.
Currently add_entries processor adds new fields to an Event. We need the ability to add entries to event's metadata.

Describe the solution you'd like
Suggest the following change to add_entries processor

processor
   - add_entries:                                                                                                                                      
         entries:                                                                                                                                        
         - metadata_key: "message_len"                                                                                                                                    
            value: 10

or

processor
   - add_entries:                                                                                                                                      
         entries:                                                                                                                                        
         - metadata_key: "message_len"                                                                                                                                    
            value_expression: `length(/message)`

Describe alternatives you've considered (Optional)
A clear and concise description of any alternative solutions or features you've considered.

Additional context
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