Add cost summary to client.py

[yiranwu0]

Why are these changes needed?

Add cost summary to client.py.
Including calculations for two types of cost:

• actual cost incurred excluding compeletions retrieved from caching.
• cost of all completions using the instance of client, including caching.

Note: cost from completions that doesn't pass a filter is also included.

Minor fix: store cost before caching.

Related issue number

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: 45 lines in your changes are missing coverage. Please review.

Comparison is base (5c92fb3) 28.00% compared to head (d852377) 41.47%.

Files	Patch %	Lines
autogen/oai/client.py	31.81%	41 Missing and 4 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #812       +/-   ##
===========================================
+ Coverage   28.00%   41.47%   +13.46%     
===========================================
  Files          27       27               
  Lines        3524     3578       +54     
  Branches      803      860       +57     
===========================================
+ Hits          987     1484      +497     
+ Misses       2465     1954      -511     
- Partials       72      140       +68     

Flag	Coverage Δ
unittests	41.36% <31.81%> (+13.41%)	⬆️

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[qingyun-wu]

Could you fix the test error?

[qingyun-wu]

The location of the added notebook link is not right. I plan to fix that in another PR #851

[yiranwu0]

The location of the added notebook link is not right. I plan to fix that in another PR #851

Got it. Updated for test error.