Initialize Client.llm_tracker on all codepaths.

[mbarnathan]

Prevents an issue with the check in stop_instrumenting().
To properly test this, I also needed to add a parameter to override the behavior of singleton.

📥 Pull Request

📘 Description
Initialize llm_tracker to None when instrument_llm_calls is False.
Add "allow_multiple_instances" kwargs to suppress singleton decorator so the client may be tested.

🔄 Related Issue (if applicable)
#256

🎯 Goal
Fix an issue that may occur when Crew and Autogen packages exist but you aren't using them.
(In my case, it popped up in a secondary file's __ main __ section)

🧪 Testing
Added a unit test; passed on tox.

[areibman]

This is awesome-- Adding @bboynton97 as a reviewer, as there might be some conflicts with the new session management strategy

[siyangqiu]

Thanks for making this PR, but due to a number of updates to AgentOps, I think this PR is no longer necessary to achieve what you are looking for:

LlmTracker is only disabled by default if you have crewai or autogen in sys.modules rather than simply installed on your system. Furthermore, the instrument_llm_calls parameter of agentops.init() should now have precedence over the default disable behavior, so you can always just set that option to True regardless of packages being used (though it may still result in double logging of LLM calls)