update docs on how to get metadata

There is a recent issue from the community (https://github.com/elastic/apm-agent-nodejs/issues/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.