Use Distinct instead of Set for map keys

[MrAlias]

As discussed in the last SIG meeting, this is a follow up to #4985. It changes our code to follow out documentation and use the Distinct type as a map key.

It also clarifies the documentation of the Set and Distinct types. There aren't any plans to change the equivalence of Sets the the documentation need to be made explicit to communicate to users the possibility.

Optimization of using Distinct as a map key are to follow (#5028).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.5%. Comparing base (da2949b) to head (8bc3fb8).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #5027   +/-   ##
=====================================
  Coverage   83.4%   83.5%           
=====================================
  Files        238     238           
  Lines      15754   15757    +3     
=====================================
+ Hits       13146   13159   +13     
+ Misses      2318    2309    -9     
+ Partials     290     289    -1     

Files	Coverage Δ
attribute/set.go	78.3% <ø> (ø)
...metric/internal/aggregate/exponential_histogram.go	97.0% <100.0%> (+<0.1%)	⬆️
sdk/metric/internal/aggregate/histogram.go	100.0% <100.0%> (ø)
sdk/metric/internal/aggregate/lastvalue.go	100.0% <100.0%> (ø)
sdk/metric/internal/aggregate/limit.go	100.0% <100.0%> (ø)
sdk/metric/internal/aggregate/sum.go	100.0% <100.0%> (ø)

... and 2 files with indirect coverage changes