ref(sessions) Change sessions interval calculation to align with metric intervals

[RaduW]

This PR changes the interval calculation for the sessions api to align to the interval calculation of metrics api.

In short, given a start_time, end_time and an interval the calculated intervals are going to:

• start at start_time if start_time is aligned with interval or at the first time before start_time that is aligned with interval.
• end at end_time if end_time is aligned with interval or at the first time after end_time that is aligned with interval.

resolves #59686

[codecov]

Codecov Report

Merging #60466 (f57bb19) into master (10beb79) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head f57bb19 differs from pull request most recent head 85de224. Consider uploading reports for the commit 85de224 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #60466      +/-   ##
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- Coverage   80.89%   80.89%   -0.01%     
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  Files        5185     5185              
  Lines      227508   227491      -17     
  Branches    38244    38237       -7     
==========================================
- Hits       184035   184020      -15     
  Misses      37854    37854              
+ Partials     5619     5617       -2     

Files	Coverage Δ
src/sentry/snuba/sessions_v2.py	94.25% <100.00%> (-0.29%)	⬇️

... and 16 files with indirect coverage changes

[Swatinem]

lgtm

[Swatinem]

as mentioned in chat, the previous hack / NOTE related to snuba caching was there to have fresher data.

In this example, the end extends past the "current" (frozen) time. Because of the caching based on the end timestamp, you would only get fresh data once every 5 minutes (the cache time back when I implemented that).
With the "cache busting", effectively truncating the end to the current time, you would get a more uptodate view of the most recent "now" interval.

[RaduW]

Thanks for clarifying