Should RESTful service in Golang include Client interface?

If I develop Booking REST service in Golang (i.e., in package booking). Is it a "GO way" to create BookingClient interface (backed up by struct) with business operations allowed, so that clients of my restful service would use BookingClient (imported from package booking) instead of sending http requests directly?

In general, no – if you provide a client in a particular language it'd only be a convenience, so (some) users can use your API easier. This of course assumes your client is well designed. I wouldn't provide merely an interface in Go just to indicate a set of possible API calls. This would be beneficial to a very narrow range of audience, probably for people developing a client for your API themselves, in programming language which just happened to be the same as implementation of your server. And even then they might not really like the idea of using the interface (e.g. they might only need a specific set of methods).

If you want to provide a client for your API, go ahead, do it, but separate it from the actual server (different package, maybe even different repo). In general one develops APIs over HTTP to allow for wide range of clients to access it, which could be written in any language. Instead of providing some interfaces I would invest my time in writing a good documentation.

In my opinion the answer to your question, assuming there is no more context provided, should be no different if you asked yourself if you should provide a client in, say, Python. The whole situation might change though if, for example, your API is used internally by your company and you develop mainly in Go.

It's usually preferable to do this, and most companies do, but provide documentation for working directly with the API. The main use case for that is people working with different languages than the ones you intended.

You can have a look at a new RESTful framework I wrote, that includes infrastructure to automatically compile clients with Go templates, although I haven't gotten to writing a Go client compiler. If you want to write one it would be greatly appreciated https://github.com/EverythingMe/vertex

Testing is important in Go, so writing testable code is something you should do. If you use direct http requests you will have a harder time writing unit tests, compared to using a mocked struct.

Is there any reason to use a Client rather than calling the functions that call the REST endpoints? It's usually harder to mock a bigger thing, such as a Client struct, rather than a group of small functions.

You should put the client at booking.Client to avoid repeating yourself (booking.BookingClient) and maybe rename Client to something more descriptive.