Simplify start_trace by setting a default param

Currently we require `Oboe::API.start_trace` to pass a value for an X-Trace value.  This is unneeded and can default to `nil`.

So calling `start_trace`without having an xtrace value will simply be called as:

```ruby
Oboe::API.start_trace('layer_name') do
  # work
end
```