update docs on how to get metadata

[david-luna]

There is a recent issue from the community (#4449) asking for and API to get metadata. The answer was that the addMetadataFilter is the right API for it but its usage is not straight forward.

We need to update the documentation with an example of this scenario so users have a reference on how to do it. There is no section in the docs for examples so a good place for it would be as part of the agent API documentation.