Cleanup interaction of exemplar and aggregation

[dashpole]

Follow-up to https://github.com/open-telemetry/opentelemetry-go/pull/5861. This is an attempt to:

• Limit the API surface of the aggregate package
• Try to use predefined types (e.g. exemplar.Filter) over custom functions where possible.
• Avoid using nil, and use No-Ops where it makes sense

This makes aggregate.NewFilteredExemplarReservoir no longer exported, removes the aggregate.FilteredExemplarReservoir interface, and removes the aggregate.dropReservoir.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 84.6%. Comparing base (bf6a7e1) to head (e626e3f). Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #5899     +/-   ##
=======================================
- Coverage   84.6%   84.6%   -0.1%     
=======================================
  Files        272     271      -1     
  Lines      22864   22855      -9     
=======================================
- Hits       19347   19337     -10     
- Misses      3172    3173      +1     
  Partials     345     345             

see 6 files with indirect coverage changes