[EXPORTER] Fix references in AttributeValueVisitor

[meastp]

Just a small fix - I think the correct fix is void operator()(nostd::string_view &&arg) (i.e. without const), at least if it was intended to overload the forwarding reference function above . Might also need void operator()(const nostd::string_view &arg) if both variants should be covered.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.67%. Comparing base (497eaf4) to head (f06d14b).
Report is 96 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2985      +/-   ##
==========================================
+ Coverage   87.12%   87.67%   +0.56%     
==========================================
  Files         200      190      -10     
  Lines        6109     5855     -254     
==========================================
- Hits         5322     5133     -189     
+ Misses        787      722      -65     

Files	Coverage Δ
...ude/opentelemetry/exporters/ostream/common_utils.h	100.00% <ø> (ø)

... and 109 files with indirect coverage changes

[marcalff]

LGTM, thanks for the fix.

Note that this code is only used for C++11.