Fix OpenTelemetry internal loading

[marcotc]

This PR fixes two issues:

• When loading datadog/opentelemetry before ddtrace, tracing/contrib files are not loaded.
• There are two default OpenTelemetry propagation getters, text_map_getter and rack_env_getter, but we only supported text_map_getter before. All our propagation code works correctly with rack_env_getter, so we allow it during configuration.

Motivation:

These two issues were found during the setup of https://github.com/DataDog/opentelemetry-demo/pull/42/files#diff-6f62dcc2d3fa2ec6fa2c3529d3d9357ae69c771fca911e87e2e982a45b50fa71

Additional Notes:

How to test the change?

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8fd4b30) 98.24% compared to head (c533905) 98.24%.
Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3400   +/-   ##
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  Coverage   98.24%   98.24%           
=======================================
  Files        1254     1254           
  Lines       73345    73346    +1     
  Branches     3435     3435           
=======================================
+ Hits        72057    72059    +2     
+ Misses       1288     1287    -1     

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