Race condition in MetricsTracer causes AttributeError: 'NoneType' object has no attribute 'status'

#### Environment details

- OS type and version: Linux (Debian 12 on GKE)
- Python version: 3.11.1
- pip version: 24.0
- `google-cloud-spanner` version: 3.61.0

#### Steps to reproduce

1. Use google-cloud-spanner with SQLAlchemy (`sqlalchemy-spanner` 1.17.2) in a multi-threaded environment (e.g., FastAPI with uvicorn workers)
2. Perform concurrent database operations (SELECT queries) under load
3. The error occurs intermittently

#### Code example

The bug is a race condition in `google/cloud/spanner_v1/metrics/metrics_interceptor.py`:

```python
def intercept(self, invoked_method, request_or_iterator, call_details):
# ...
SpannerMetricsTracerFactory.current_metrics_tracer.set_method(method_name)
SpannerMetricsTracerFactory.current_metrics_tracer.record_attempt_start()
response = invoked_method(request_or_iterator, call_details)
SpannerMetricsTracerFactory.current_metrics_tracer.record_attempt_completion() # BUG: current_metrics_tracer may have been replaced by another thread
# ...
```

`SpannerMetricsTracerFactory.current_metrics_tracer` is a **class variable** shared across all threads. Between `record_attempt_start()` and `record_attempt_completion()`, another thread can call `MetricsCapture.__enter__()` which replaces `current_metrics_tracer` with a new instance:

```python
# In metrics_capture.py __enter__():
SpannerMetricsTracerFactory.current_metrics_tracer = factory.create_metrics_tracer()
```

When `record_attempt_completion()` is called, it operates on a different `MetricsTracer` instance that hasn't had `record_attempt_start()` called, so `current_op.current_attempt` is `None`.

**Suggested fix:** Make `current_metrics_tracer` thread-local instead of a class variable:

```python
import threading

class SpannerMetricsTracerFactory(MetricsTracerFactory):
_metrics_tracer_factory: "SpannerMetricsTracerFactory" = None
_thread_local = threading.local()

@property
def current_metrics_tracer(cls):
return getattr(cls._thread_local, 'metrics_tracer', None)

@current_metrics_tracer.setter
def current_metrics_tracer(cls, value):
cls._thread_local.metrics_tracer = value
```

**Additional bug:** There's a typo in `spanner_metrics_tracer_factory.py` line 93:
```python
cls._metrics_tracer_factory.enabeld = enabled # Should be "enabled" not "enabeld"
```

#### Stack trace
```
File "/usr/local/lib/python3.11/site-packages/google/cloud/spanner_v1/metrics/metrics_interceptor.py", line 148, in intercept
SpannerMetricsTracerFactory.current_metrics_tracer.record_attempt_completion()

File "/usr/local/lib/python3.11/site-packages/google/cloud/spanner_v1/metrics/metrics_tracer.py", line 332, in record_attempt_completion
self.current_op.current_attempt.status = status

AttributeError: 'NoneType' object has no attribute 'status'
```

**Workaround:** Disable built-in metrics by calling `SpannerMetricsTracerFactory(enabled=False)` before any Spanner operations.