sdk: shut down span processors automatically

The BatchExportSpanProcessor is an asynchronous span processor that uses a
worker thread to call the different exporters.  Before this commit applications
had to shut down the span processor explicitely to guarantee that all the spans
were summited to the exporters, this was not very intuitive for the users.

This commit removes that limitation by implementing the tracer's __del__
method and an atexit hook.  According to __del__'s documentation [1] it is
possible that sometimes it's not called, for that reason the atexit hook is
also used to guarantee that the processor is shut down in all the cases.

[1] https://docs.python.org/3/reference/datamodel.html#object.__del__

examples/trace/client.py

@@ -39,4 +39,3 @@
 tracer.add_span_processor(span_processor)
 
 response = requests.get(url="http://127.0.0.1:5000/")
-span_processor.shutdown()

examples/trace/server.py

@@ -52,4 +52,3 @@ def hello():
 
 if __name__ == "__main__":
     app.run(debug=True)
-    span_processor.shutdown()

ext/opentelemetry-ext-azure-monitor/examples/client.py

@@ -27,4 +27,3 @@
 tracer.add_span_processor(span_processor)
 
 response = requests.get(url="http://127.0.0.1:5000/")
-span_processor.shutdown()

ext/opentelemetry-ext-azure-monitor/examples/server.py

@@ -41,4 +41,3 @@ def hello():
 
 if __name__ == "__main__":
     app.run(debug=True)
-    span_processor.shutdown()

ext/opentelemetry-ext-jaeger/README.rst

@@ -54,10 +54,6 @@ gRPC is still not supported by this implementation.
     with tracer.start_as_current_span('foo'):
         print('Hello world!')
 
-    # shutdown the span processor
-    # TODO: this has to be improved so user doesn't need to call it manually
-    span_processor.shutdown()
-
 The `examples <./examples>`_ folder contains more elaborated examples.
 
 References

ext/opentelemetry-ext-jaeger/examples/jaeger_exporter_example.py

@@ -45,7 +45,3 @@
         time.sleep(0.2)
 
     time.sleep(0.1)
-
-# shutdown the span processor
-# TODO: this has to be improved so user doesn't need to call it manually
-span_processor.shutdown()

opentelemetry-sdk/src/opentelemetry/sdk/trace/__init__.py

@@ -13,6 +13,7 @@
 # limitations under the License.
 
 
+import atexit
 import logging
 import random
 import threading
@@ -333,6 +334,13 @@ def __init__(
         self._current_span_slot = Context.register_slot(slot_name)
         self._active_span_processor = MultiSpanProcessor()
         self.sampler = sampler
+        self._atexit_hanlder = atexit.register(
+            self._active_span_processor.shutdown
+        )
+
+    def __del__(self):
+        atexit.unregister(self._atexit_hanlder)
+        self._active_span_processor.shutdown()
 
     def get_current_span(self):
         """See `opentelemetry.trace.Tracer.get_current_span`."""