Separate CLS from Tracer and make it optional

[hekike]

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

Continues local storage has an overhead which maybe makes sense in applications where the developer doesn't control the instrumentation and it should happen automatically but is unnecessary in services with explicit instrumentation.

Read more about common issues with CLS in othiym23/node-continuation-local-storage#59.

Describe the solution you'd like

My proposal would be to remove CLS usage from the CoreTracer class and create a new class CoreTracerCLS which extends the CoreTracer and enhances it with CLS. Then we could expose the CoreTracer and keep CoreTracerCLS the default Tracer to avoid breaking change.

Describe alternatives you've considered

A bigger change would be to keep the Core OpenCensus Node module explicit and completely CLS and monkey patching free and create a new module that does the automatic instrumentation via the pure core module with CLS and require hooks.

Additional context

This proposal is slightly related to these changes which give more control to the developer:

• Add support for nested spans #437
• Consolidating Span and RootSpan to allow Spans to recursively have children #441

I'm happy to help with the implementation and open a PR.

This Issue was created based on our conversation with @bogdandrutu, @mayurkale22, @kjin, @ofrobots, and @kenashcraft

[mayurkale22]

Thanks for filling the issue, feel free to shoot the PR.

[mayurkale22]

@hekike Can we close this one via #484?

[hekike]

Yes, thank you!