Option to disable async_hooks/cls

[dicarlo2]

NB: Before opening a feature request against this repo, consider whether the feature should/could be implemented in other the OpenCensus libraries in other languages. If so, please open an issue on opencensus-specs first.

Is your feature request related to a problem? Please describe.
Basically same comment as #462.

Describe the solution you'd like
CLS to be optional.

Describe alternatives you've considered
Nada.

Additional context

opencensus-node/packages/opencensus-core/src/internal/cls-ah.ts

Line 35 in 2ae76c3

asyncHook.createHook({ init, before, destroy }).enable();

means that async hooks are initialized regardless of whether or not the tracer implementation that uses them is imported by the user. Please move the enable call to outside of module scope.