Merge otel into one file and add package doc

[rakyll]

No need to split a tiny package into multiple files. Also adding package doc for the otel package.

[codecov]

Codecov Report

Merging #1176 into master will increase coverage by 0.3%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #1176     +/-   ##
========================================
+ Coverage    77.9%   78.2%   +0.3%     
========================================
  Files         135     135             
  Lines        7192    7163     -29     
========================================
- Hits         5604    5603      -1     
+ Misses       1344    1317     -27     
+ Partials      244     243      -1     

Impacted Files	Coverage Δ
sdk/trace/batch_span_processor.go	78.5% <0.0%> (-1.1%)	⬇️
api/metric/numberkind_string.go	50.0% <0.0%> (+25.0%)	⬆️
api/metric/kind_string.go	50.0% <0.0%> (+33.3%)	⬆️
sdk/export/metric/exportkind_string.go	100.0% <0.0%> (+35.7%)	⬆️
label/type_string.go	50.0% <0.0%> (+37.5%)	⬆️