Document f in RegisterCallback needs to be concurrent safe

[pellared]

Make the concurrent safety requirement harder to miss.

See:

opentelemetry-go/metric/meter.go

Lines 174 to 175 in 6b262b4

	// The function needs to be concurrent safe.
	type Callback func(context.Context, Observer) error

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.5%. Comparing base (5e69812) to head (911420a).
Report is 1576 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4251     +/-   ##
=======================================
- Coverage   83.5%   83.5%   -0.1%     
=======================================
  Files        181     181             
  Lines      14162   14162             
=======================================
- Hits       11832   11830      -2     
- Misses      2103    2105      +2     
  Partials     227     227             

see 1 file with indirect coverage changes

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