module/apmzerolog: add Writer.MinLevel

[axw]

Add Writer.MinLevel, which controls the minimum
level of logs to send as errors to Elastic APM.
MinLevel must be >= zerolog.ErrorLevel, otherwise
zerolog.ErrorLevel will be used as the minimum.

Closes #636

[codecov-io]

Codecov Report

Merging #641 into master will increase coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #641     +/-   ##
=========================================
+ Coverage   87.78%   87.88%   +0.1%     
=========================================
  Files         123      123             
  Lines        7582     7587      +5     
=========================================
+ Hits         6656     6668     +12     
+ Misses        820      816      -4     
+ Partials      106      103      -3

Impacted Files	Coverage Δ
module/apmzerolog/writer.go	72.72% <100%> (+3.57%)	⬆️
tracer.go	89.69% <0%> (+0.32%)	⬆️
transport/http.go	95.41% <0%> (+1.14%)	⬆️

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