…) (#62764)

* Fix broken `dag_processing.total_parse_time` metric (#62128)

DagFileProcessorManager has been emitting a nonsense value for
`dag_processing.total_parse_time` since 8774f28, which reversed the
order in which `emit_metrics` and `prepare_file_queue` (then called
`prepare_file_path_queue`) were called.

As `prepare_file_path_queue` was responsible for resetting the value of
`self._parsing_start_time`, the assumption made by `emit_metrics` was
that it would be called once the file queue had been cleared, but
crucially before `prepare_file_queue` was called to refill the queue.

Additionally, there was no guarantee that we'd parsed any files at all
since the last time the metric was emitted. If no work was due, we'd
gladly emit near-zero metrics every time around the while loop.

I've rearranged things in such a way that I hope will be harder to
accidentally break in future:

- `self._parsing_start_time` may be reset whenever files are added to
  the queue, if it was not set already.

- metrics are emitted when `prepare_file_queue` is called -- when the
  queue is empty -- but only if `self._parsing_start_time` is set,
  meaning only if we've actually parsed any files since the last time
  metrics were emitted.

Together, this means we should now emit metrics once per parsing loop.
I've added a test which fails on main and passes on this branch.

(cherry picked from commit 57a7c64)

* fix static checks

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Co-authored-by: Nick Stenning <nick@whiteink.com>