[Documentation] Missing documentation about how to configure the underlying HttpClient for IDownstreamApi

[EnricoMassone]

Documentation related to component

This request for missing documentation is related with the usage of IDownstreamApi interface.

Please check all that apply

• typo
• documentation doesn't exist
• documentation needs clarification
• error(s) in the example
• needs an example

Description of the issue

I'm currently unable to find a documentation and a sample about how to customize the underlying HttpClient when using the IDownstreamApi interface to call a downstream api. This problem was originally discussed in #1740 and the suggested solution is to register an HttpClient with the very same name of the registered downstream api.

Since the original author of the issue confirmed that this solution works and there was no new guideline about this topic, my suggestion is to add an official statement and possibly an example to the documentation.