Fix FileStream scanning_total to include sync next-file open time (#20627)

## 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
I tested by reading CSV files via AWS S3.

---------

Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>

Author: RatulDawar

datafusion/datasource/src/file_stream.rs

@@ -127,7 +127,15 @@ impl FileStream {
                         self.file_stream_metrics.files_opened.add(1);
                         // include time needed to start opening in `start_next_file`
                         self.file_stream_metrics.time_opening.stop();
-                        let next = self.start_next_file().transpose();
+                        let next = {
+                            let scanning_total_metric = self
+                                .file_stream_metrics
+                                .time_scanning_total
+                                .metrics
+                                .clone();
+                            let _timer = scanning_total_metric.timer();
+                            self.start_next_file().transpose()
+                        };
                         self.file_stream_metrics.time_scanning_until_data.start();
                         self.file_stream_metrics.time_scanning_total.start();