Fix FileStream scanning_total to include sync next-file open time

[RatulDawar]

Summary

• include synchronous start_next_file() / FileOpener::open() setup time in time_elapsed_scanning_total
• keep existing time_opening and scanning timers lifecycle intact
• avoid timer overlap by scoping the temporary timer before calling time_scanning_total.start()

Details

In FileStreamState::Open, start_next_file() is invoked before time_scanning_total.start(). If open() performs synchronous work before returning the future, that time was previously unaccounted for in time_elapsed_scanning_total.

This change wraps the start_next_file() call in a scoped timer on the same time_scanning_total metric so the missing segment is recorded.

Fixes #20571

Validation

• cargo test -p datafusion-datasource with_limit_at_middle_of_batch -- --nocapture