Add documentation on runtime.Start for scheduling metrics

[anuraaga]

Had completely missed scheduling metrics are separate from what otherwise seems like the full entrypoint to the package. I think a link to the other entrypoint would be very helpful here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.1%. Comparing base (e49abb5) to head (7da9bf4).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #6686     +/-   ##
=======================================
- Coverage   73.1%   73.1%   -0.1%     
=======================================
  Files        191     191             
  Lines      15858   15858             
=======================================
- Hits       11604   11598      -6     
- Misses      3921    3925      +4     
- Partials     333     335      +2     

Files with missing lines	Coverage Δ
instrumentation/runtime/runtime.go	76.2% <ø> (ø)

... and 1 file with indirect coverage changes