Add auto instrument shim

[TonyCTHsu]

What does this PR do?

Add an auto instrument shim

In Gemfile

gem 'ddtrace', require: 'ddtrace/auto_instrument'
# Is the same 
gem 'ddtrace', require: 'datadog/auto_instrument'

Motivation:

Smoother upgrade for 2.0

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.24%. Comparing base (0ca28c1) to head (73f7eab).
Report is 7 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3535   +/-   ##
=======================================
  Coverage   98.24%   98.24%           
=======================================
  Files        1275     1275           
  Lines       75329    75329           
  Branches     3559     3559           
=======================================
  Hits        74005    74005           
  Misses       1324     1324           

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