Added sample for configuring warmup requests. (GoogleCloudPlatform#1918)

* Added sample for configuring warmup requests.

* Remove unused region tag.

* Minor fix.

Author: chenyumic

appengine/standard_python37/warmup/app.yaml

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+runtime: python37
+
+inbound_services:
+- warmup

appengine/standard_python37/warmup/main.py

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+# Copyright 2018 Google LLC.
+#
+# 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.
+
+# [START gae_python37_warmup_app]
+from flask import Flask
+
+
+app = Flask(__name__)
+
+
+@app.route('/')
+def main():
+    return 'Hello World!'
+
+
+@app.route('/_ah/warmup')
+def warmup():
+    # Handle your warmup logic here, e.g. set up a database connection pool
+    return '', 200, {}
+
+
+if __name__ == '__main__':
+    # This is used when running locally only. When deploying to Google App
+    # Engine, a webserver process such as Gunicorn will serve the app. This
+    # can be configured by adding an `entrypoint` to app.yaml.
+    app.run(host='127.0.0.1', port=8080, debug=True)
+# [END gae_python37_warmup_app]

appengine/standard_python37/warmup/main_test.py

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+# Copyright 2018 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.
+
+import main
+
+
+def test_index():
+    main.app.testing = True
+    client = main.app.test_client()
+
+    r = client.get('/')
+    assert r.status_code == 200
+    assert 'Hello World' in r.data.decode('utf-8')
+
+
+def test_warmup():
+    main.app.testing = True
+    client = main.app.test_client()
+
+    r = client.get('/')
+    assert r.status_code == 200

appengine/standard_python37/warmup/requirements.txt

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+flask==1.0.2