Feature Request: Return mock object from `get_tracer()` when inactive

[aradzu10]

Feature Request: Return mock object from get_tracer() when inactive

Details

When running without viztracer, viztracer.get_tracer() returns None. This causes crashes when instrumentation code is present in the application but the tracer is not active.

It should instead return a mock or no-op object so that code execution continues without AttributeError.

Example:
The following code currently crashes if viztracer is not running:

# AttributeError: 'NoneType' object has no attribute 'log_event'
with get_tracer().log_event("scope"):
    pass

[gaogaotiantian]

That would be a breaking change. get_tracer(noop_object=True) (or some other smart name) could work. The original idea was for the users to explicitly check if tracer exists. However, in this specific case, that code would be much more difficult to write.

[aradzu10]

I'm not sure the noop_object=True is neccessery.
From my experience, if I didn't turn on the tracer, I don't really care of it.
Because the tracer is like "write only" object, and we shouldn't use it to "read" info, changing to the noop (when not initializing) should be just fine.

In my opinion, we don't really need to state "noop_object", and the fact we didn't initialize the tracer is enough.
But, I'm ok with this solution.