Track memory used by DoH buffers

[pgporada]

Fixes #71

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e284f50) 5.23% compared to head (677514f) 5.23%.

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Additional details and impacted files

@@          Coverage Diff          @@
##            main     #72   +/-   ##
=====================================
  Coverage   5.23%   5.23%           
=====================================
  Files          1       1           
  Lines        191     191           
=====================================
  Hits          10      10           
  Misses       181     181           

Flag	Coverage Δ
unbound_exporter_tests	5.23% <ø> (ø)

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[mcpherrinm]

want to add this to the expected metrics list in the integration test? https://github.com/letsencrypt/unbound_exporter/blob/main/integration_test.go#L47