Add ERROR_RATIO_4XX to default set of metrics

[bergetp]

Emit ERROR_RATIO_4XX metric by default and update documentation.

[Codecov]

Codecov Report

Merging #305 into 1.x will increase coverage by 0.06%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                1.x     #305      +/-   ##
============================================
+ Coverage     72.79%   72.85%   +0.06%     
+ Complexity      736      735       -1     
============================================
  Files           148      148              
  Lines          3113     3113              
  Branches        174      174              
============================================
+ Hits           2266     2268       +2     
+ Misses          805      802       -3     
- Partials         42       43       +1     

Impacted Files	Coverage Δ	Complexity Δ
...spotify/apollo/metrics/semantic/MetricsConfig.java	95.45% <ø> (ø)	9.00 <0.00> (ø)
...tify/apollo/request/TrackedOngoingRequestImpl.java	78.57% <0.00%> (-7.15%)	4.00% <0.00%> (-1.00%)
...main/java/com/spotify/apollo/core/ServiceImpl.java	91.03% <0.00%> (+1.03%)	33.00% <0.00%> (ø%)

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[klaraward]

Just a note that the corresponding 5xx metrics were added in this PR - #301

[lepistone]

Thanks!