move google-cloud-trace to extras (#162)

Author: jpoehnelt

docs/trace/usage.rst

@@ -26,6 +26,10 @@ Installation & basic usage
         pip install opencensus-trace
         pipenv install opencensus-trace
 
+        # install the Stackdriver exporter by including the extras package
+        pip install opencensus-trace[stackdriver]
+        pipenv install opencensus-trace[stackdriver]
+
 2. Initialize a tracer for application:
 
     .. code:: python

setup.py

@@ -17,8 +17,12 @@
 import io
 from setuptools import setup, find_packages
 
+extras = {
+    "stackdriver": ['google-cloud-trace>=0.17.0, <0.18dev']
+}
+
 install_requires = [
-    'google-cloud-trace>=0.17.0, <0.18dev',
+    'google-api-core >= 0.1.1, < 0.2.0dev',
 ]
 
 setup(
@@ -43,6 +47,7 @@
     include_package_data=True,
     long_description=open('README.rst').read(),
     install_requires=install_requires,
+    extras_require=extras,
     license='Apache-2.0',
     packages=find_packages(),
     namespace_packages=[],