Change version detection mecanism

[maxday]

Description

pkg_resources is quite heavy and slows down cold start when dd-trace-py is used in a AWS Lambda environment.
This PR uses the write_to mecanism to write this version to a file (and defaulting to pkg_resources in case of an unexpected error)

Checklist

• Added to the correct milestone.
• Tests provided or description of manual testing performed is included in the code or PR.
• Library documentation is updated.
• Corp site documentation is updated (link to the PR).

[brettlangdon]

big picture looks good

[Kyle-Verhoog]

@maxday @brettlangdon what are the plans for how/when the _version file will be generated?

[maxday]

@maxday @brettlangdon what are the plans for how/when the _version file will be generated?

config.py is doing it thanks to write_to (the file was not committed when you made the comment)

[brettlangdon]

@maxday do you have performance/profile results with this change? how much of an improvement does it make?

[maxday]

@maxday do you have performance/profile results with this change? how much of an improvement does it make?

@brettlangdon Yes : over 10 runs, result is the average

Before : 853ms
With this PR : 665ms

This is a massive win for our cold start duration.

(Full disclosure, I had to comment the import in debug.py as well since it also imports pkg_resources)
EDIT : no need to edit debug.py manually anymore since #2803 has been merged

[Kyle-Verhoog]

awesome! lgtm with just a tweak to the test cases

[Kyle-Verhoog]

oh sweet - that was easy!

[codecov-commenter]

Codecov Report

Merging #2802 (a436d4a) into master (d6bbd13) will increase coverage by 0.00%.
The diff coverage is 90.69%.

@@           Coverage Diff           @@
##           master    #2802   +/-   ##
=======================================
  Coverage   84.87%   84.87%           
=======================================
  Files         592      595    +3     
  Lines       43384    43421   +37     
=======================================
+ Hits        36820    36855   +35     
- Misses       6564     6566    +2     

Impacted Files	Coverage Δ
ddtrace/internal/debug.py	35.29% <0.00%> (-1.97%)	⬇️
tests/tracer/test_version.py	89.65% <89.65%> (ø)
ddtrace/__init__.py	100.00% <100.00%> (+11.11%)	⬆️
ddtrace/version.py	100.00% <100.00%> (ø)
tests/tracer/_version.py	100.00% <100.00%> (ø)

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[maxday]

I have moved tests under tests/tracer and they are now correctly run during CI : https://app.circleci.com/pipelines/github/DataDog/dd-trace-py/10831/workflows/0f140edc-eb28-44de-a268-db2cce5b5360/jobs/820560?invite=true#step-104-1458