Use span.data instead of measurements for token usage

[antonpirker]

Store AI token usage in span.data instead of deprecated measurements.

In relay there is already code in place that copies the data from the deprecated span.measurements to span.data and uses span.data for calculating the cost of token usage. So this PR can be deployed in a minor without risk.

See also relay PR: getsentry/relay#4768

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.73%. Comparing base (5709b25) to head (a2f9f0b).
Report is 2 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4567      +/-   ##
==========================================
+ Coverage   80.72%   80.73%   +0.01%     
==========================================
  Files         156      156              
  Lines       16481    16481              
  Branches     2801     2801              
==========================================
+ Hits        13304    13306       +2     
- Misses       2295     2296       +1     
+ Partials      882      879       -3     

Files with missing lines	Coverage Δ
sentry_sdk/ai/monitoring.py	86.30% <100.00%> (ø)

... and 3 files with indirect coverage changes