Add semantic attributes for HTTP content length.

[anuraaga]

open-telemetry/opentelemetry-specification#641

[codecov]

Codecov Report

Merging #1376 into master will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #1376      +/-   ##
============================================
+ Coverage     91.94%   92.02%   +0.08%     
  Complexity      888      888              
============================================
  Files           114      114              
  Lines          3178     3186       +8     
  Branches        262      262              
============================================
+ Hits           2922     2932      +10     
+ Misses          174      173       -1     
+ Partials         82       81       -1     

Impacted Files	Coverage Δ	Complexity Δ
...telemetry/trace/attributes/SemanticAttributes.java	100.00% <100.00%> (ø)	1.00 <0.00> (ø)
...telemetry/sdk/trace/export/BatchSpanProcessor.java	95.94% <0.00%> (+1.35%)	8.00% <0.00%> (ø%)

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