docs: fix server metrics example

[BonifacioCalindoro]

The existing examples/get_server_metrics.py version has 2 problems:

• the iso-formatted dates need a timezone in order to work with the API
• Object of type Metrics is not JSON serializable anymore.

So i added an explicit UTC timezone for the datetime objects, and got rid of the json.dumps() at the end of the script.

[jooola]

Hey, thanks for the contribution :)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.57%. Comparing base (14ecce2) to head (c9d03e1).
⚠️ Report is 43 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #547   +/-   ##
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  Coverage   96.57%   96.57%           
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  Files          64       64           
  Lines        2945     2945           
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  Hits         2844     2844           
  Misses        101      101           

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