chore(trace-explorer): Make sample rate optional for trace explorer

[Zylphrex]

This is likely the cause of there only being 1 span per trace. Let's make it optional so we can test turning it off and see how that affects results.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.67%. Comparing base (604a22e) to head (05d0a94).
Report is 45 commits behind head on master.

❗ Current head 05d0a94 differs from pull request most recent head 3a216ef. Consider uploading reports for the commit 3a216ef to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #69052       +/-   ##
===========================================
+ Coverage   54.00%   79.67%   +25.67%     
===========================================
  Files        5433     6427      +994     
  Lines      228412   285073    +56661     
  Branches    39284    49096     +9812     
===========================================
+ Hits       123360   227140   +103780     
+ Misses     104687    57567    -47120     
- Partials      365      366        +1     

Files	Coverage Δ
src/sentry/api/endpoints/organization_traces.py	60.71% <100.00%> (ø)
src/sentry/options/defaults.py	100.00% <ø> (ø)

... and 2228 files with indirect coverage changes