Rake:Require instrumented task list

[marcotc]

Release notes

Users of the Rake instrumentation will have to explicitly list the tasks that they want instrumented. This was done to prevent memory bloat issues that can arise from instrumenting long-running Rake tasks.

From today's configuration:

Datadog.configure do |c|
  c.tracing.instrument :rake
end

The list of desired Rake tests to instrument should be added:

Datadog.configure do |c|
  c.tracing.instrument :rake, tasks: ['task1', 'task2', ...]
end

Not providing this list will effectively instrument no Rake tasks.

What does this PR do?

This PR requires the Rake instrumentation to receive an explicit list of task to be instrumented. This list can be a list of String task names, or other values that can be stringified (e.g. it's common to declare Rake tasks with Symbol names).

Motivation

Long-running Rake tasks, those measured in days, can accumulate a very large active trace in memory. This normally won't be flushed until the application terminates.

Such traces can cause catastrophic memory accumulation, possibly triggering an out-of-memory process shutdown.

Examples of such cases can be found in #2045.

Additional Notes

I chose to no require users to have all their Rake loaded before invoking Datadog.configure: our current documentation suggests that users configure ddtrace before Rake task definition, so I chose to maintain compatibility and such flexibility.
Also, the implementation is not very complicated: there's a small performance overhead on non-instrumented tasks, but it boils down to a Set#include? call.
The dynamic implementation also allows for dynamic reconfiguration of the tasks: configuration list. The eager approach will either "patch" the Rake tasks and never be able to "unpatch", or it will implement the dynamic approach in this PR as well as selective patching.

Testing for the unrelated when tracing is disabled case was a bit misleading around tracer #shutdown! calls: it was asserting on a Datadog::Tracing.trace instance, but another instance of the tracer was used during testing. This PR fixes that.

How to test the change?

Unit test coverage around Rake tasks is already pretty good. I added the relevant cases introduced in this PR.

[TonyCTHsu]

Looks good!