Fix start timestamp in auto root span

[znerol]

Problem: The start time in AutoRootSpan is recorded in milliseconds instead of nanoseconds.

Proposal: Use ClockInterface::NANOS_PER_SECOND instead of a magic value.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.56%. Comparing base (fd654b9) to head (e1410ce).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1413      +/-   ##
============================================
- Coverage     73.73%   73.56%   -0.17%     
  Complexity     2683     2683              
============================================
  Files           387      387              
  Lines          7672     7975     +303     
============================================
+ Hits           5657     5867     +210     
- Misses         2015     2108      +93     

Flag	Coverage Δ
8.1	73.31% <100.00%> (-0.10%)	⬇️
8.2	73.44% <100.00%> (-0.22%)	⬇️
8.3	73.44% <100.00%> (-0.25%)	⬇️
8.4	73.49% <100.00%> (-0.15%)	⬇️

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Files with missing lines	Coverage Δ
src/SDK/Trace/AutoRootSpan.php	85.71% <100.00%> (+0.60%)	⬆️

... and 207 files with indirect coverage changes

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