feat(discover): Allows insight metrics to be used with the avg function in the discover metrics dataset

[edwardgou-sentry]

Allows insights metrics (eg: span.self_time) to be used in the metrics avg function

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

:white_check_mark: All tests successful. No failed tests found.

Additional details and impacted files

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- Coverage   90.49%   78.27%   -12.22%     
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  Files        3099     6824     +3725     
  Lines      193934   303942   +110008     
  Branches        0    52287    +52287     
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+ Hits       175504   237918    +62414     
- Misses      18430    59635    +41205     
- Partials        0     6389     +6389