Clarify logs when a DAG is being processed in DPM

[amoghrajesh]

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The "still being processed" debug log in the DAG file processor manager was logging
processor.start_time which is a time.monotonic() value, and honestly that is confusing while debugging

For example, it could lead to misreading 2637 as
"this file has been processing for 2637 seconds" when it is actually the monotonic clock value at process start).

I propose to add the elapsed duration (now - processor.start_time) alongside the start time (not removing it if someone has processes that rely on it), so the log now reads:

File path my_dag.py is still being processed (started: 2637.46, duration: 12.34s)

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[Copilot]

Pull request overview

This PR improves the debug logging in the DAG file processor manager to make it clearer how long a DAG file has been processing, avoiding confusion caused by logging a raw time.monotonic() start value.

Changes:

• Updates the “still being processed” debug log message to include an elapsed duration (time.monotonic() - processor.start_time).
• Switches log formatting to fixed-precision floats for start and duration.