Add missing exponential_filter export and tests

[christophfroehlich]

Someone forgot to check if the tests run at all 👼

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.15%. Comparing base (9756cb7) to head (4bb9395).

Additional details and impacted files

@@               Coverage Diff               @@
##           ros2-master     #260      +/-   ##
===============================================
+ Coverage        72.75%   75.15%   +2.40%     
===============================================
  Files               25       26       +1     
  Lines             1101     1131      +30     
  Branches            89       89              
===============================================
+ Hits               801      850      +49     
+ Misses             252      231      -21     
- Partials            48       50       +2     

Flag	Coverage Δ
unittests	75.15% <ø> (+2.40%)	⬆️

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see 6 files with indirect coverage changes