Security: Query request object logged at INFO level may expose sensitive filter values

## Summary

In `metricflow/engine/metricflow_engine.py` (line 523):

```python
logger.info(LazyFormat("Starting query request", mf_request=mf_request))
```

The full `MetricFlowQueryRequest` object is logged at INFO level. This object may contain WHERE filter values that include PII or sensitive business data (customer names, email addresses, financial thresholds, etc.).

INFO-level logs are typically retained in production log aggregation systems and may be accessible to a broader set of operators than intended.

## Suggested Fix

- Change to `logger.debug()` so it only appears when explicitly enabled
- Or redact/summarize the request object before logging at INFO (e.g., log only the metric names and request ID, not filter values)

## Severity

**Medium** — information disclosure via logs. Impact depends on what data flows through WHERE filters and who has access to production logs.