refactor: address review comments on centralized routing helpers

Author: hebaalazzeh

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

@@ -144,15 +144,10 @@ class {{ service.client_name }}(metaclass={{ service.client_name }}Meta):
     """{{ service.meta.doc|rst(width=72, indent=4) }}{% if service.version|length %}
     This class implements API version {{ service.version }}.{% endif %}"""
 
-    @staticmethod
-    def _get_default_mtls_endpoint(api_endpoint: Optional[str]) -> Optional[str]:
-        """Converts api endpoint to mTLS endpoint."""
-        return routing.get_default_mtls_endpoint(api_endpoint)
-
     # Note: DEFAULT_ENDPOINT is deprecated. Use _DEFAULT_ENDPOINT_TEMPLATE instead.
     DEFAULT_ENDPOINT = {% if service.host %}"{{ service.host }}"{% else %}None{% endif %}
 
-    DEFAULT_MTLS_ENDPOINT = _get_default_mtls_endpoint.__func__(  # type: ignore
+    DEFAULT_MTLS_ENDPOINT = routing.get_default_mtls_endpoint(
         DEFAULT_ENDPOINT
     )
 
@@ -370,7 +365,19 @@ class {{ service.client_name }}(metaclass={{ service.client_name }}Meta):
         universe_domain: str,
         use_mtls_endpoint: str,
     ) -> str:
-        """Return the API endpoint used by the client."""
+        """Return the API endpoint used by the client.
+
+        Args:
+            api_override (Optional[str]): The API endpoint override. If specified, this is always
+                the return value of this function and the other arguments are not used.
+            client_cert_source (Union[Callable[[], Tuple[bytes, bytes]], None]): The client certificate source used by the client.
+            universe_domain (str): The universe domain used by the client.
+            use_mtls_endpoint (str): How to use the mTLS endpoint, which depends also on the other parameters.
+                Possible values are "always", "auto", or "never".
+
+        Returns:
+            str: The API endpoint to be used by the client.
+        """
         return routing.get_api_endpoint(
             api_override,
             client_cert_source,
@@ -383,7 +390,18 @@ class {{ service.client_name }}(metaclass={{ service.client_name }}Meta):
 
     @staticmethod
     def _get_universe_domain(client_universe_domain: Optional[str], universe_domain_env: Optional[str]) -> str:
-        """Return the universe domain used by the client."""
+        """Return the universe domain used by the client.
+
+        Args:
+            client_universe_domain (Optional[str]): The universe domain configured via the client options.
+            universe_domain_env (Optional[str]): The universe domain configured via the "GOOGLE_CLOUD_UNIVERSE_DOMAIN" environment variable.
+
+        Returns:
+            str: The universe domain to be used by the client.
+
+        Raises:
+            ValueError: If the universe domain is an empty string.
+        """
         return routing.get_universe_domain(
             client_universe_domain,
             universe_domain_env,

packages/gapic-generator/gapic/templates/tests/unit/gapic/%name_%version/%sub/test_%service.py.j2

@@ -156,22 +156,6 @@ def set_event_loop():
             asyncio.set_event_loop(None)
 
 
-def test__get_default_mtls_endpoint():
-    api_endpoint = "example.googleapis.com"
-    api_mtls_endpoint = "example.mtls.googleapis.com"
-    sandbox_endpoint = "example.sandbox.googleapis.com"
-    sandbox_mtls_endpoint = "example.mtls.sandbox.googleapis.com"
-    non_googleapi = "api.example.com"
-    custom_endpoint = ".custom"
-
-    assert {{ service.client_name }}._get_default_mtls_endpoint(None) is None
-    assert {{ service.client_name }}._get_default_mtls_endpoint(api_endpoint) == api_mtls_endpoint
-    assert {{ service.client_name }}._get_default_mtls_endpoint(api_mtls_endpoint) == api_mtls_endpoint
-    assert {{ service.client_name }}._get_default_mtls_endpoint(sandbox_endpoint) == sandbox_mtls_endpoint
-    assert {{ service.client_name }}._get_default_mtls_endpoint(sandbox_mtls_endpoint) == sandbox_mtls_endpoint
-    assert {{ service.client_name }}._get_default_mtls_endpoint(non_googleapi) == non_googleapi
-    assert {{ service.client_name }}._get_default_mtls_endpoint(custom_endpoint) == custom_endpoint
-
 def test__read_environment_variables():
     assert {{ service.client_name }}._read_environment_variables() == (False, "auto", None)