ConfigureNLog should not be used together with UseNLog

[snakefoot]

UseNLog together with ConfigureNLog will generate duplicate logging (builder.AddNLog will register a second time).

See also:

https://stackoverflow.com/questions/33613971/nlog-logs-error-twice-in-same-file
https://stackoverflow.com/questions/42589841/how-can-i-stop-nlog-from-double-logging

I guess registering an ILoggerFactory as singleton (That ignores any calls to AddProvider) would also solve the issue (Making it more difficult to configure wrong). See also #235

[codecov]

Codecov Report

Merging #257 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #257   +/-   ##
=====================================
  Coverage      57%    57%           
=====================================
  Files          31     31           
  Lines         407    407           
  Branches       92     92           
=====================================
  Hits          233    233           
  Misses        138    138           
  Partials       36     36

Impacted Files	Coverage Δ
...eb.AspNetCore/Internal/RequestAccessorExtension.cs	100% <100%> (ø)	⬆️

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