attribute: add Value.String() and deprecate Emit; update usages

[sonalgaud12]

#8144

Add String() string to attribute.Value implementing fmt.Stringer, with the same output as Emit(). Deprecate Emit in favor of String. Update go.opentelemetry.io/otel/exporters/prometheus to use Value.String() instead of Value.Emit().

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.7%. Comparing base (3b18b21) to head (8031d15).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #8154     +/-   ##
=======================================
+ Coverage   81.8%   82.7%   +0.9%     
=======================================
  Files        308     308             
  Lines      23801   28511   +4710     
=======================================
+ Hits       19482   23600   +4118     
- Misses      3942    4534    +592     
  Partials     377     377             

Files with missing lines	Coverage Δ
attribute/value.go	91.7% <100.0%> (-1.0%)	⬇️
exporters/prometheus/exporter.go	90.8% <100.0%> (+<0.1%)	⬆️

... and 260 files with indirect coverage changes

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[pellared]

The issue is blocked by #8142.

[sonalgaud12]

@pellared Closed it for now