add cost calculation to client

[yiranwu0]

Why are these changes needed?

Add cost calculation to client in v0.2

Related issue number

#614

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation 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.

[codecov-commenter]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (b30ead2) 27.61% compared to head (66ba07e) 37.21%.

Files	Patch %	Lines
autogen/oai/client.py	21.42%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #769      +/-   ##
==========================================
+ Coverage   27.61%   37.21%   +9.60%     
==========================================
  Files          27       27              
  Lines        3480     3493      +13     
  Branches      789      791       +2     
==========================================
+ Hits          961     1300     +339     
+ Misses       2447     2073     -374     
- Partials       72      120      +48     

Flag	Coverage Δ
unittests	37.16% <26.66%> (+9.54%)	⬆️

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[joshkyh]

Great work! Quite a few people are asking for cost reports, it's great to have this feature (back)!

[qingyun-wu]

The code LGTM. Just one suggestion about documentation: We should make this feature discoverable with proper documentation or demonstration in a notebook.

[yiranwu0]

The code LGTM. Just one suggestion about documentation: We should make this feature discoverable with proper documentation or demonstration in a notebook.

Got it. I just added the print cost in the documentation. I will add a notebook about cost calculation in the next PR when implementing cost summary of a client instance.

[afourney]

The code LGTM. Just one suggestion about documentation: We should make this feature discoverable with proper documentation or demonstration in a notebook.

I'll add it to the Testbed as well, once it has progressed to the point where we're computing costs for agents and group chats.