feat: add google.api_core.version_header (#638)

* feat: add google.api_core.gapic_v1.version_header

* fix mypy

* clarify comment

google/api_core/version_header.py

@@ -0,0 +1,29 @@
+# Copyright 2024 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.
+
+API_VERSION_METADATA_KEY = "x-goog-api-version"
+
+
+def to_api_version_header(version_identifier):
+    """Returns data for the API Version header for the given `version_identifier`.
+
+    Args:
+        version_identifier (str): The version identifier to be used in the
+            tuple returned.
+
+    Returns:
+        Tuple(str, str): A tuple containing the API Version metadata key and
+            value.
+    """
+    return (API_VERSION_METADATA_KEY, version_identifier)

tests/unit/test_version_header.py

@@ -0,0 +1,23 @@
+# Copyright 2024 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.
+
+import pytest
+
+from google.api_core import version_header
+
+
+@pytest.mark.parametrize("version_identifier", ["some_value", ""])
+def test_to_api_version_header(version_identifier):
+    value = version_header.to_api_version_header(version_identifier)
+    assert value == (version_header.API_VERSION_METADATA_KEY, version_identifier)