Add Prometheus target availability plugin

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Codecov Report

Merging #16 (dbff81f) into main (4b9df55) will decrease coverage by 0.39%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##             main      #16      +/-   ##
==========================================
- Coverage   89.61%   89.21%   -0.40%     
==========================================
  Files           9       10       +1     
  Lines         231      241      +10     
==========================================
+ Hits          207      215       +8     
- Misses         12       13       +1     
- Partials       12       13       +1     

Impacted Files	Coverage Δ
plugins/prometheus/targets/availability/plugin.go	80.00% <80.00%> (ø)

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