Allow user to instrument a library without enabling tracing (using the NoopTracer)

[mayurkale22]

No description provided.

[mayurkale22]

#277

[bogdandrutu]

In java we call this BlankSpan, because people may use this even for a request that they don't want to trace.

[mayurkale22]

Changed NoopSpan to BlankSpan...

[mayurkale22]

/cc @liyanhui1228 for review

[liyanhui1228]

Is the NoopSpan is an improvement of the NullContextManager? If so, we could remove the NullContextManager class as it won't be used anywhere else. This could be done in a separate PR. And be sure to sync up this branch with master branch before merging.

[mayurkale22]

@liyanhui1228 Thanks for the review

[odeke-em]

Thanks for the fix @mayurkale22 and @bogdandrutu @liyanhui1228 for the review! I just have one minor nit: in the future please refer to issues in the commit message so that Github can auto-close them on merge to the main branch e.g.

trace: make a NoopTracer that drops spans instead of crashing

....[Rest of message]....

Fixes #277

that way I'd have gotten the notification 11 days ago on it being closed with a PR instead of almost 2 weeks later :)