Support for metadata and types in expressions

[dlvenable]

Is your feature request related to a problem? Please describe.

The Data Prepper event model has support for metadata which includes type information.

Data Prepper expression syntax allows for accessing data using JSON Pointer syntax. For example, /status_code. However, there is no mechanism to access Event Metadata.

Some pipeline authors would like to perform pipeline routing based on event type. Some others might want to filter based on tags (#629).

Describe the solution you'd like

Design and implement a syntax that allows for accessing any EventMetadata.

Additional context

The discussion around supporting Event tags offers a proposal for getTags() in this comment.

[kkondaka]

Working on adding getMetadata() function to DataPrepper Expressions which looks up a key in the event's metadata attributes. For event type, getEventType() function can be added in future.
Examples - getMetadata("key1") getMetadata("key1/key2"), getMetadata("key1/key2/key3")when "/" present, the lookup is done nested way. This is similar to the way key lookup is done in events. The keys could start with "/" (as ingetMetadata("/key1") getMetadata("/key1/key2"), getMetadata("/key1/key2/key3") ) and they are treated same as the keys without preceding "/"

[kkondaka]

Fixed by PR #2690