Ensure Rails testing is considered a development environment

[marcotc]

What does this PR do?

This PR detects if a Rails application is running in development or test mode, and uses this information to inform ddtrace internally in order improve logging and background worker management.

Motivation

Currently, we consider local development Rails console as a production environment, which is incorrect.

Additional Notes

This checking ensures staging, which is a common non-stardard Rails environment is considered "not development" by ddtrace.

How to test the change?

There are unit test covering this change.

[anmarchenko]

I think we need to go forward with that because there are still issues with telemetry and RC being reported.

Do you think we would need to add a check for Cucumber in test? method?

[marcotc]

Do you think we would need to add a check for Cucumber in test? method?

That's a good idea, I'll open a separate PR.

[codecov-commenter]

Codecov Report

Merging #3062 (f648160) into master (74cb196) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3062   +/-   ##
=======================================
  Coverage   98.16%   98.16%           
=======================================
  Files        1283     1283           
  Lines       73898    73918   +20     
  Branches     3421     3421           
=======================================
+ Hits        72542    72565   +23     
+ Misses       1356     1353    -3     

Files Changed	Coverage Δ
lib/datadog/core/environment/execution.rb	100.00% <100.00%> (ø)
spec/datadog/core/environment/execution_spec.rb	96.11% <100.00%> (+0.66%)	⬆️

... and 3 files with indirect coverage changes

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