tests/unit: test_metrics_emission_with_failure_attempt fails due to lack of "intercept"

This is the failure output

```shell
_________________________________________________ test_metrics_emission_with_failure_attempt _________________________________________________

patched_client =

def test_metrics_emission_with_failure_attempt(patched_client):
instance = patched_client.instance("test-instance")
database = instance.database("example-db")
factory = SpannerMetricsTracerFactory()

assert factory.enabled

transport = database.spanner_api._transport
metrics_interceptor = transport._metrics_interceptor
> original_intercept = metrics_interceptor.intercept
E AttributeError: 'NoneType' object has no attribute 'intercept'

tests/unit/test_metrics.py:58: AttributeError
------------------------------------------------------------- Captured log setup -------------------------------------------------------------
WARNING opentelemetry.metrics._internal:__init__.py:864 Overriding of current MeterProvider is not allowed
----------------------------------------------------------- Captured log teardown ------------------------------------------------------------
WARNING opentelemetry.metrics._internal:__init__.py:864 Overriding of current MeterProvider is not allowed
============================================================== warnings summary ==============================================================
```