Fix the threshold precision issue

[guanw]

Signed-off-by: Jude Wang judew@uber.com

Which problem is this PR solving?

• Precision Issue with Downsampling Writer Threshold #1664

Short description of the changes

• It's potential for conversion from an uint64 traceid to float64 number to lose precision.

[codecov]

Codecov Report

Merging #1665 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1665      +/-   ##
==========================================
+ Coverage   98.71%   98.71%   +<.01%     
==========================================
  Files         193      193              
  Lines        9240     9243       +3     
==========================================
+ Hits         9121     9124       +3     
  Misses         91       91              
  Partials       28       28

Impacted Files	Coverage Δ
storage/spanstore/downsampling_writer.go	100% <100%> (ø)	⬆️

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