fix(google-cloud-spanner): address compatibility issues with spanner experimental (#16593)

Compatibility with experimental host broke post the major refactor in
https://github.com/googleapis/python-spanner/pull/1509/changes.

These minor changes fixed the compatibility. Verified with below
integration tests run

Integration test successful:
```
export SPANNER_EXPERIMENTAL_HOST=localhost:15000
python -m pytest -v -s  --disable-warnings tests/system/
```

Author: sagnghos

packages/google-cloud-spanner/google/cloud/spanner_v1/_async/client.py

@@ -313,6 +313,7 @@ def __init__(
             self._client_certificate = client_certificate
             self._client_key = client_key
             credentials = AnonymousCredentials()
+            disable_builtin_metrics = True
         elif isinstance(credentials, AnonymousCredentials):
             self._emulator_host = self._client_options.api_endpoint
 
@@ -645,6 +646,7 @@ def instance(
             self._emulator_host,
             labels,
             processing_units,
+            self._experimental_host,
         )
 
     @CrossSync.convert

packages/google-cloud-spanner/google/cloud/spanner_v1/client.py

@@ -276,6 +276,7 @@ def __init__(
             self._client_certificate = client_certificate
             self._client_key = client_key
             credentials = AnonymousCredentials()
+            disable_builtin_metrics = True
         elif isinstance(credentials, AnonymousCredentials):
             self._emulator_host = self._client_options.api_endpoint
         super(Client, self).__init__(
@@ -547,6 +548,7 @@ def instance(
             self._emulator_host,
             labels,
             processing_units,
+            self._experimental_host,
         )
 
     def list_instances(self, filter_="", page_size=None):

packages/google-cloud-spanner/tests/system/_async/conftest.py

@@ -35,7 +35,10 @@ def spanner_client():
 
         credentials = AnonymousCredentials()
         return spanner_v1.AsyncClient(
-            project=_helpers.EXPERIMENTAL_HOST_PROJECT,
+            use_plain_text=_helpers.USE_PLAIN_TEXT,
+            ca_certificate=_helpers.CA_CERTIFICATE,
+            client_certificate=_helpers.CLIENT_CERTIFICATE,
+            client_key=_helpers.CLIENT_KEY,
             credentials=credentials,
             experimental_host=_helpers.EXPERIMENTAL_HOST,
         )