Add Python bindings for accessing ExecutionMetrics #1379

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

DataFusion Python currently provides execution metrics only through the explain(analyze=True) output, which displays metrics as formatted console text. There is no structured Python API to programmatically access per-operator metrics such as output_rows, elapsed_compute, spill_count, etc.

Describe the solution you'd like
Expose a structured Python API to access execution metrics after running a query
Describe alternatives you've considered
N/A

Additional context
This mirrors the existing Rust API in datafusion::physical_plan::metrics and makes it accessible from Python. The metrics would only be populated after execution, matching DataFusion's semantics where metrics are collected during query execution.