Fix SpanRef::set_attributes mutability

[jtescher]

SpanRefs already synchronize access so the reference can be &self instead of &mut self.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (f42c11d) 55.1% compared to head (fc4f71f) 55.1%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #1038   +/-   ##
=====================================
  Coverage   55.1%   55.1%           
=====================================
  Files        149     149           
  Lines      18143   18143           
=====================================
  Hits       10006   10006           
  Misses      8137    8137           

Impacted Files	Coverage Δ
opentelemetry-api/src/trace/context.rs	21.3% <0.0%> (ø)

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