add env var to disable runtime tracing

[EItanya]

Why are these changes needed?

Recently a PR merged to enable GENAI semantic convention tracing, however, when using component loading it's not currently possible to disable the runtime tracing.

This PR allows disabling said tracing using an ENV var

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[ekzhu]

Could you also update the user guide and API doc of SingleThreadedAgentRuntime as well?

[codecov]

Codecov Report

Attention: Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.

Project coverage is 79.70%. Comparing base (89927ca) to head (b18f14d).

Files with missing lines	Patch %	Lines
...togen-core/src/autogen_core/_telemetry/_tracing.py	57.14%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6681      +/-   ##
==========================================
- Coverage   79.71%   79.70%   -0.02%     
==========================================
  Files         232      232              
  Lines       17323    17329       +6     
==========================================
+ Hits        13809    13812       +3     
- Misses       3514     3517       +3     

Flag	Coverage Δ
unittests	79.70% <57.14%> (-0.02%)	⬇️

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