Adding metadata sample

Change-Id: I9876e6badd5dfda2c142b153a9ec1c54ca5d4a79

compute/metadata/README.md

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+# Compute Engine Metadata Samples
+
+These samples demonstrate interacting with the Compute Engine metadata service. These samples must be run on Compute Engine.
+
+<!-- auto-doc-link -->
+<!-- end-auto-doc-link -->

compute/metadata/main.py

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+#!/usr/bin/env python
+
+# Copyright 2016 Google Inc. All Rights Reserved.
+#
+# 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.
+
+"""Example of using the Compute Engine API to watch for maintenance notices.
+
+For more information, see the README.md under /compute.
+"""
+
+# [START all]
+
+import time
+
+import requests
+
+
+METADATA_URL = "http://metadata.google.internal/computeMetadata/v1/"
+METADATA_HEADERS = {'Metadata-Flavor': 'Google'}
+
+
+def wait_for_maintenance(callback):
+    url = METADATA_URL + 'instance/maintenance-event'
+    last_in_maintenance = False
+    # [START hanging_get]
+    last_etag = 0
+
+    while True:
+        r = requests.get(
+            url,
+            params={'last_etag': last_etag},
+            headers=METADATA_HEADERS)
+
+        print(r)
+
+        # During maintenance the service can return a 503, so these should
+        # be retried.
+        if r.status_code == 503:
+            time.sleep(1)
+            continue
+        # [END hanging_get]
+
+        last_etag = r.headers['etag']
+
+        if r.text == 'MIGRATE_ON_HOST_MAINTENANCE':
+            in_maintenance = True
+        else:
+            in_maintenance = False
+
+        if in_maintenance != last_in_maintenance:
+            last_in_maintenance = in_maintenance
+            callback(in_maintenance)
+
+
+def maintenance_callback(status):
+    if status:
+        print('Undergoing host maintenance')
+    else:
+        print('Finished host maintenance')
+
+
+def main():
+    wait_for_maintenance(maintenance_callback)
+
+
+if __name__ == '__main__':
+    main()
+# [END all]

compute/metadata/main_test.py

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+# Copyright 2016, Google, Inc.
+# 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 main
+import mock
+
+
+@mock.patch('main.requests')
+def test_wait_for_maintenance(requests_mock):
+    # Response 1 is a host maintenance event.
+    response1_mock = mock.Mock()
+    response1_mock.status_code = 200
+    response1_mock.text = 'MIGRATE_ON_HOST_MAINTENANCE'
+    response1_mock.headers = {'etag': 1}
+    # Response 2 is the end of the event.
+    response2_mock = mock.Mock()
+    response2_mock.status_code = 200
+    response2_mock.text = 'NONE'
+    response2_mock.headers = {'etag': 2}
+    # Response 3 is a 503
+    response3_mock = mock.Mock()
+    response3_mock.status_code = 503
+
+    requests_mock.get.side_effect = [
+        response1_mock, response2_mock, response3_mock, response2_mock,
+        StopIteration()]
+
+    callback_mock = mock.Mock()
+
+    try:
+        main.wait_for_maintenance(callback_mock)
+    except StopIteration:
+        pass

compute/metadata/requirements.txt

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+requests==2.9.1