feature: record duration using monotonic time

[sfozz]

Adds nanosecond-resolution request timing to the access log and switches the underlying measurement to a monotonic
clock.

What's new

• New %(N)s access-log atom for request time in nanoseconds. This lets gunicorn line up directly with other systems
  (tracing, metrics pipelines, various proxies) that already record duration at nanosecond resolution, so they agree
  without any unit conversion.
• Monotonic timing for request duration is measured using time.monotonic_ns() instead of datetime.now(). Using the monotonic timer means that duration is recorded precisely and without the possibility of inaccurate records due to the system clock changing.

Details

• request_time is now an integer count of nanoseconds rather than a timedelta. All existing atoms are derived from it
  with plain integer arithmetic:
  • T: seconds
  • M: milliseconds
  • D: microseconds
  • N: nanoseconds (new)
  • L: decimal seconds (to microsecond precision)
• The statsd instrumentation is updated to compute its millisecond duration from the same nanosecond value.
• Applies consistently across the sync, async, and gthread workers, plus the error-handling path.
• Tests updated to pass nanosecond integers and to cover the new N atom.

Implements #3398

[sfozz]

I've cleaned this up and made the change more intentional.

• Cleaned up the atoms so that the math is easier to follow
• updated the tests to handle the change

[sfozz]

Oh I see what you mean now... I've just tested extending the gunicorn.glogging.Logger which would have been a breaking change because of this PR!

I've updated the PR description to more acuuratly cover the purpose of the change.