Add a tracing custom fetch example #4

Background:
The docs show how to pass a custom fetch. A tracing example would help users
wrap request execution for simple logging or instrumentation while still using
the default transport.

Proposed change:
Add an example that defines a FetchLike, records method and URL, then delegates
to the runtime fetch.

Acceptance criteria:

• The example imports FetchLike.
• The example delegates to fetch(input, init).
• The example does not depend on a specific logging library.
• The example explains that full execution control should use Transport
  instead.

Files likely involved:

• docs/examples.md
• docs/guides/rest-requests.md
• src/transport/types.ts for source reference only