Add WSGI middleware

[Kyle-Verhoog]

Integration

This PR adds a WSGI middleware to be used to trace any WSGI Python web application.

Note: in cases of interrupted streaming requests, which can lead to broken pipe connections and therefore GeneratorExit exceptions in the response spans, the decision was made to prevent the GeneratorExit exceptions from being propagated to the top-level request spans. These exceptions will still be present in the wsgi.response spans.

Links

• Corp docs PR:
• Manual test app: python: add django 3 app trace-examples#106

Checklist

• Usage and configuration documentation added in __init__.py.
• Corp docs are updated.
• Entry added to release notes using reno.
• Distributed tracing propagation is implemented (if applicable).
• Global configuration
  • Inherits DD_SERVICE (if applicable).
  • Environment variables are provided for config options.
  • ddtrace.config.
  • Make distributed tracing configurable.
• Instance configuration
  • Pin overriding.
  • Service name override (if applicable).
• HTTP (if applicable)
  • 500-level responses are tagged as errors.
  • Determine an appropriate URL to use for the resource
• Web (if applicable)
  • Request headers specified in the config are stored.
  • Use SpanTypes.WEB for request span of trace.
• Tests
  • Tests are provided for all of the above.
  • Tests are added to CI (.circleci/config.yml).
  • Functionality is maintained from original library.
  • All Python versions that the library supports are tested.
  • All relevant library versions (including the latest) are tested.
• Handle builtins.GeneratorExit errors with streaming requests (expected behaviour?) probably have to use span ignored exceptions.
• Use the set_http_meta helper to set http tags
• Add equivalent to span_modifier from the ASGI middleware

[codecov-io]

Codecov Report

Merging #1808 (4181db7) into master (e933d24) will increase coverage by 0.04%.
The diff coverage is 95.90%.

@@            Coverage Diff             @@
##           master    #1808      +/-   ##
==========================================
+ Coverage   90.90%   90.94%   +0.04%     
==========================================
  Files         555      559       +4     
  Lines       38819    39063     +244     
==========================================
+ Hits        35287    35527     +240     
- Misses       3532     3536       +4     

Impacted Files	Coverage Δ
tests/tracer/test_tracer.py	98.36% <ø> (ø)
ddtrace/contrib/wsgi/wsgi.py	88.76% <88.76%> (ø)
ddtrace/contrib/wsgi/__init__.py	100.00% <100.00%> (ø)
tests/conftest.py	100.00% <100.00%> (ø)
tests/contrib/wsgi/test_wsgi.py	100.00% <100.00%> (ø)
tests/profiling/gevent_fork.py	61.53% <0.00%> (-30.77%)	⬇️
ddtrace/provider.py	87.50% <0.00%> (-8.34%)	⬇️
tests/commands/test_runner.py	0.00% <0.00%> (-8.06%)	⬇️
ddtrace/utils/wrappers.py	38.23% <0.00%> (-5.89%)	⬇️
ddtrace/__init__.py	88.23% <0.00%> (-5.89%)	⬇️
... and 29 more

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[Kyle-Verhoog]

lgtm!