[I] Enable accessing Message in TableView [pulsar-client-go]

["petermnhull (via GitHub)" <gi...@apache.org>]

petermnhull opened a new issue, #1176:
URL: https://github.com/apache/pulsar-client-go/issues/1176

   **Is your feature request related to a problem? Please describe.**
   I have a use case for processing messages in a TableView where I need to be able to access the metadata (e.g. publication time) for each message, for example to check if the message for a key is "out-of-date" or recent.
   
   Currently this is not possible as the TableView implementation only provides the message as encoded by the SchemaValueType and discards all metadata.
   
   **Describe the solution you'd like**
   I would like to introduce two new methods to the TableView interface for accessing the associated `pulsar.Message` for each key:
   1. `Message(key string) Message` - access the `pulsar.Message` for a given key (as an alternative to `Get`)
   2.`Messages() map[string]Message` - access all the stored `pulsar.Message` objects for each key (as an alternative to `Entries`)
   
   I propose to do this by changing how the messages are processed - instead of storing the encoded payload in `processMessage`, instead we store a map of map[string]Message. Backwards compatibility will be maintained by always returning the payload in the existing methods.
   
   **Describe alternatives you've considered**
   Alternatives include:
   There are no current alternatives for accessing message metadata while using TableView.
   
   Instead of changing any of the storage implementation, we can add a new map which additionally stores a mapping from key to `pulsar.Message`, and only use this map for the two new methods. I initially decided against this to avoid duplicating the logic for storing messages.
   
   **Additional context**
   NA.


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