[SDTEST-408] multi threaded code coverage support for datadog_cov

[anmarchenko]

What does this PR do?
Fixes an issue when per test code coverage for ITR ignored background threads. This led to the coverage being incomplete in the most cases as Rails uses threading for background jobs testing for example.

Now by default code coverage is collected for all threads, not only the current thread of the test. I added a parameter that will allow the library to switch back to track single thread in a less probable situation where threaded parallel test runner is used.

Motivation
Reports of incomplete code coverage when running Rails tests

Additional notes
This is built on top of PR that improves code coverage tool performance: #171

How to test the change?
Unit tests are provided

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.86%. Comparing base (b04133e) to head (19e5d8b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #189      +/-   ##
==========================================
+ Coverage   98.85%   98.86%   +0.01%     
==========================================
  Files         228      229       +1     
  Lines       10082    10172      +90     
  Branches      465      468       +3     
==========================================
+ Hits         9967    10057      +90     
  Misses        115      115              

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[ivoanjo]

👍 Seems reasonable!

Given that dd-trace-rb also has a bunch of background threads, would it make sense to exclude them by default?

E.g. by looking at their names for instance (all of our threads should be named with the Datadog:: prefix).