🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

google/cloud/spanner_admin_database_v1/services/database_admin/transports/grpc.py

@@ -64,7 +64,7 @@ def __init__(
         credentials: Optional[ga_credentials.Credentials] = None,
         credentials_file: Optional[str] = None,
         scopes: Optional[Sequence[str]] = None,
-        channel: Optional[grpc.Channel] = None,
+        channel: Optional[Union[grpc.Channel, Callable[..., grpc.Channel]]] = None,
         api_mtls_endpoint: Optional[str] = None,
         client_cert_source: Optional[Callable[[], Tuple[bytes, bytes]]] = None,
         ssl_channel_credentials: Optional[grpc.ChannelCredentials] = None,
@@ -84,14 +84,17 @@ def __init__(
                 credentials identify the application to the service; if none
                 are specified, the client will attempt to ascertain the
                 credentials from the environment.
-                This argument is ignored if ``channel`` is provided.
+                This argument is ignored if a ``channel`` instance is provided.
             credentials_file (Optional[str]): A file with credentials that can
                 be loaded with :func:`google.auth.load_credentials_from_file`.
-                This argument is ignored if ``channel`` is provided.
+                This argument is ignored if a ``channel`` instance is provided.
             scopes (Optional(Sequence[str])): A list of scopes. This argument is
-                ignored if ``channel`` is provided.
-            channel (Optional[grpc.Channel]): A ``Channel`` instance through
-                which to make calls.
+                ignored if a ``channel`` instance is provided.
+            channel (Optional[Union[grpc.Channel, Callable[..., grpc.Channel]]]):
+                A ``Channel`` instance through which to make calls, or a Callable
+                that constructs and returns one. If set to None, ``self.create_channel``
+                is used to create the channel. If a Callable is given, it will be called
+                with the same arguments as used in ``self.create_channel``.
             api_mtls_endpoint (Optional[str]): Deprecated. The mutual TLS endpoint.
                 If provided, it overrides the ``host`` argument and tries to create
                 a mutual TLS channel with client SSL credentials from
@@ -101,11 +104,11 @@ def __init__(
                 private key bytes, both in PEM format. It is ignored if
                 ``api_mtls_endpoint`` is None.
             ssl_channel_credentials (grpc.ChannelCredentials): SSL credentials
-                for the grpc channel. It is ignored if ``channel`` is provided.
+                for the grpc channel. It is ignored if a ``channel`` instance is provided.
             client_cert_source_for_mtls (Optional[Callable[[], Tuple[bytes, bytes]]]):
                 A callback to provide client certificate bytes and private key bytes,
                 both in PEM format. It is used to configure a mutual TLS channel. It is
-                ignored if ``channel`` or ``ssl_channel_credentials`` is provided.
+                ignored if a ``channel`` instance or ``ssl_channel_credentials`` is provided.
             quota_project_id (Optional[str]): An optional project to use for billing
                 and quota.
             client_info (google.api_core.gapic_v1.client_info.ClientInfo):
@@ -132,7 +135,7 @@ def __init__(
         if client_cert_source:
             warnings.warn("client_cert_source is deprecated", DeprecationWarning)
 
-        if channel:
+        if isinstance(channel, grpc.Channel):
             # Ignore credentials if a channel was passed.
             credentials = False
             # If a channel was explicitly provided, set it.
@@ -173,7 +176,9 @@ def __init__(
         )
 
         if not self._grpc_channel:
-            self._grpc_channel = type(self).create_channel(
+            # initialize with the provided callable or the default channel
+            channel_init = channel or type(self).create_channel
+            self._grpc_channel = channel_init(
                 self._host,
                 # use the credentials which are saved
                 credentials=self._credentials,