Add failed_download_count metric

[FileGo]

This implements #309 . Adds blocky_failed_download_count Prometheus metric, which increments the counter for every failed attempt to download a blocking list.

[codecov]

Codecov Report

Merging #395 (ac64aae) into development (9259c5c) will increase coverage by 1.27%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           development     #395      +/-   ##
===============================================
+ Coverage        93.89%   95.17%   +1.27%     
===============================================
  Files               31       31              
  Lines             2426     2427       +1     
===============================================
+ Hits              2278     2310      +32     
+ Misses             111       80      -31     
  Partials            37       37              

Impacted Files	Coverage Δ
lists/list_cache.go	94.50% <100.00%> (+0.03%)	⬆️
resolver/caching_resolver.go	95.27% <0.00%> (-3.15%)	⬇️
redis/redis.go	89.06% <0.00%> (+27.34%)	⬆️

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