Make child-spans work as expected when using the lambda-runtime

[TethysSvensson]

Issue #, if available: N/A

Description of changes: Implementing a sub-span directly in the service_fn does not currently set the parent-span in the expected way. Example:

let func = service_fn(move |event: LambdaEvent<T>| {
    let event_span = create_event_span(outer_variable);
    inner_handler(event, state.clone()).instrument(event_span)
});

The problem is, that at the time where the create_event_span() function is called, the span returned by request_span() has not been entered, so it will be not be set as the parent.

The solution is to enter the span before calling the inner service. This also matches e.g. how tower_http does it.

By submitting this pull request

• [x] I confirm that my contribution is made under the terms of the Apache 2.0 license.
• [x] I confirm that I've made a best effort attempt to update all relevant documentation.

[TethysSvensson]

And thank you for accepting it!

Are you by any chance planning to do a release for this?

I'm asking because if you are, then I would rather wait for your release than create a work-around in our internal code.

[calavera]

I'm asking because if you are, then I would rather wait for your release than create a work-around in our internal code.

Not this week.