Provide an option for tracing all rake task

[heka1024]

Is your feature request related to a problem? Please describe.

When I tried to trace rake task, I should put all names of rake task. But my application has tons of task and it's bit annoying to list all this names.

Describe the goal of the feature

Provide an option to trace all rake task, without providing names. I think configuration can be like this.

Datadog.configure do |c|
    # Activating instrumentation
    c.tracing.instrument :rake, {service_name: "my-rake", tasks: :all } # or provide manually. like ["temp:datadog_test"]
end

Additional context

If I aplied something wrong option, please let me know 😓 . If This request makes sense, I'll make a pull request for this.