Make all fetch timestamps 0

[esantorella]

Summary: Status: Needs feedback; this is what I am most uncertain about, particularly as it interacts with storage. Once this is done, D86452716 (making _data_by_trial a Data) is easier.

Alternately we could get rid of the timestamps and have _data_by_trial: {trial_index: Data}, which would require, for legacy experiments, combining data from multiple timestamps into one upon deserialization. Constructing Data upon deserialization for experiments with data in the legacy _data_by_trial format will become necessary regardless once we give Experiment a data attribute, but I'm not sure it's worth updating the structure of _data_by_trial when we're about to change this again.

Differential Revision: D86521191

[meta-codesync[bot]]

@esantorella has exported this pull request. If you are a Meta employee, you can view the originating Diff in D86521191.

[codecov-commenter]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 96.57%. Comparing base (769e0d1) to head (ee24720).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4520      +/-   ##
==========================================
- Coverage   96.57%   96.57%   -0.01%     
==========================================
  Files         548      548              
  Lines       56447    56456       +9     
==========================================
+ Hits        54514    54520       +6     
- Misses       1933     1936       +3     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

:rocket: New features to boost your workflow:

• :snowflake: Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• :package: JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.