Adding w3c tracecontext integration test (#228)

Verifying that our tracecontext is compliant with the w3c
tracecontext reference is valuable. Adding a tox command
to verify that the TraceContext propagator adheres to the
w3c spec.

The tracecontexthttptextformat is now completely compliant with the w3c
tracecontext test suite.

.flake8

@@ -10,6 +10,7 @@ exclude =
   .svn
   .tox
   CVS
+  target
   __pycache__
   ext/opentelemetry-ext-jaeger/src/opentelemetry/ext/jaeger/gen/
   ext/opentelemetry-ext-jaeger/build/*

.isort.cfg

@@ -12,4 +12,5 @@ line_length=79
 ; )
 ; docs: https://github.com/timothycrosley/isort#multi-line-output-modes
 multi_line_output=3
+skip=target
 skip_glob=ext/opentelemetry-ext-jaeger/src/opentelemetry/ext/jaeger/gen/*

tox.ini

@@ -5,6 +5,7 @@ envlist =
     py3{4,5,6,7,8}-test-{api,sdk,example-app,ext-wsgi,ext-http-requests,ext-jaeger,opentracing-shim}
     pypy3-test-{api,sdk,example-app,ext-wsgi,ext-http-requests,ext-jaeger,opentracing-shim}
     lint
+    py37-tracecontext
     py37-{mypy,mypyinstalled}
     docs
 
@@ -111,3 +112,21 @@ changedir = docs
 
 commands =
   sphinx-build -W --keep-going -b html -T . _build/html
+
+[testenv:py37-tracecontext]
+basepython: python3.7
+deps =
+  # needed for tracecontext
+  aiohttp~=3.6
+  # needed for example trace integration
+  flask~=1.1
+  requests~=2.7
+
+commands_pre =
+  pip install -e {toxinidir}/opentelemetry-api
+  pip install -e {toxinidir}/opentelemetry-sdk
+  pip install -e {toxinidir}/ext/opentelemetry-ext-http-requests
+  pip install -e {toxinidir}/ext/opentelemetry-ext-wsgi
+
+commands =
+  {toxinidir}/scripts/tracecontext-integration-test.sh