BigQuery: provide a public API to opt out of built-in OpenTelemetry tracing

### Determine this is the right repository

- [x] I determined this is the correct repository in which to report this feature request.

### Summary of the feature request

When `opentelemetry-api` is installed and a global tracer provider is configured, `google-cloud-bigquery` automatically wraps every BQ call in a span (`BigQuery.job.begin`, `BigQuery.getQueryResults`, etc.) via `opentelemetry_tracing.create_span`. There is no documented way to opt out, so projects that want OTel for their own code but not for BQ end up with their traces drowned in BQ spans.

### Desired code experience

Any of the following would be a supported, public way to disable the library's built-in tracing:

```python
# Option A — env var
# GOOGLE_CLOUD_BIGQUERY_DISABLE_TRACING=1

# Option B — constructor argument
from google.cloud import bigquery
client = bigquery.Client(enable_tracing=False)

# Option C — honor the OTel SDK's standard opt-out
# OTEL_PYTHON_DISABLED_INSTRUMENTATIONS=google-cloud-bigquery
```

Happy to send a PR if there is a preferred approach.

### Expected results

With any of the above set, BQ client calls should not emit spans, even though `opentelemetry-api` is installed and a tracer provider is configured for the rest of the application.

### API client name and version

google-cloud-bigquery v3.38.0

### Use case

We use Logfire (OpenTelemetry-based) to trace AI-agent runs and outbound HTTP. Once the tracer provider is configured, every BQ call also emits a span, which drowns out the agent traces we actually care about. We want OTel for our own code, but not for BQ.

### Additional context

The only working workaround today is mutating the module-level `HAS_OPENTELEMETRY` flag before any BQ call:

```python
import google.cloud.bigquery.opentelemetry_tracing as _bq_otel
_bq_otel.HAS_OPENTELEMETRY = False
```

This works because `create_span` reads the global at call time and `client.py` only imports `create_span`, not the flag itself. But it relies on a non-public attribute and could silently regress on any internal refactor. Uninstalling `opentelemetry-api` is not viable when the rest of the application depends on it.

Source reference: `packages/google-cloud-bigquery/google/cloud/bigquery/opentelemetry_tracing.py` — the `HAS_OPENTELEMETRY` flag and `create_span` function.