feat: add support for reading google.api.api_version

Author: parthea

gapic/schema/wrappers.py

@@ -1734,6 +1734,17 @@ def host(self) -> str:
             return self.options.Extensions[client_pb2.default_host]
         return ''
 
+    @property
+    def api_version(self) -> str:
+        """Return the API version for this service, if specified.
+
+        Returns:
+            str: The API version for this service.
+        """
+        if self.options.Extensions[client_pb2.api_version]:
+            return self.options.Extensions[client_pb2.api_version]
+        return ''
+
     @property
     def shortname(self) -> str:
         """Return the API short name. DRIFT uses this to identify

gapic/templates/%namespace/%name_%version/%sub/services/%service/client.py.j2

@@ -102,7 +102,11 @@ class {{ service.client_name }}Meta(type):
 
 
 class {{ service.client_name }}(metaclass={{ service.client_name }}Meta):
-    """{{ service.meta.doc|rst(width=72, indent=4) }}"""
+    """{{ service.meta.doc|rst(width=72, indent=4) }}
+    {% if service.api_version|length %}
+    This class implements API version {{ service.api_version }}.
+    {% endif %}
+    """
 
     @staticmethod
     def _get_default_mtls_endpoint(api_endpoint):