add async span example

contrib/opencensus-context/examples/async_span.py

@@ -0,0 +1,50 @@
+# Copyright 2019, OpenCensus Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import asyncio
+from opencensus.common.runtime_context import RuntimeContext
+
+RuntimeContext.register_slot('current_span', None)
+
+
+class Span(object):
+    def __init__(self, name):
+        self.name = name
+        self.parent = RuntimeContext.current_span
+
+    def __repr__(self):
+        return ('{}(name={}, parent={})'.format(type(self).__name__, self.name, self.parent))
+
+    async def __aenter__(self):
+        RuntimeContext.current_span = self
+
+    async def __aexit__(self, exc_type, exc, tb):
+        RuntimeContext.current_span = self.parent
+
+
+async def main():
+    print(RuntimeContext)
+    async with Span('foo'):
+        print(RuntimeContext)
+        await asyncio.sleep(0.1)
+        async with Span('bar'):
+            print(RuntimeContext)
+            await asyncio.sleep(0.1)
+        print(RuntimeContext)
+        await asyncio.sleep(0.1)
+    print(RuntimeContext)
+
+
+if __name__ == '__main__':
+    asyncio.run(main())