[V1][Metrics] Handle preemptions

[markmc]

Part of #10582

Add a core engine PREEMPTED event.

Add the num_preemptions_total counter from v0.

Also, make preemptions reset the scheduled and first token timestamps resulting in:

  << queued timestamp >>
    [ queue interval ]
      |
      |	(possible preemptions)
      | << scheduled timestamp >>
      | << preempted timestamp >>
      | << scheduled timestamp >>
      | << new token timestamp (FIRST) >>
      | << preempted timestamp >>
      v
  << scheduled timestamp >>
    [ prefill interval ]
  << new token timestamp (FIRST) >>
    [ inter-token interval ]
  << new token timestamp >>
    [ decode interval (relative to most recent first token time)
    [ inference interval (relative to most recent scheduled time)
  << new token timestamp (FINISHED) >>

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[markmc]

pre-commit failure is a yapf failure that doesn't happen for me locally:

  File "/home/runner/.cache/pre-commit/repom9gt4aao/py_env-python3.12/lib/python3.12/site-packages/yapf_third_party/_ylib2to3/pygram.py", line 39, in <module>
    pattern_grammar = driver.load_grammar(_PATTERN_GRAMMAR_FILE)
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/.cache/pre-commit/repom9gt4aao/py_env-python3.12/lib/python3.12/site-packages/yapf_third_party/_ylib2to3/pgen2/driver.py", line 248, in load_grammar
    g.load(gp)
  File "/home/runner/.cache/pre-commit/repom9gt4aao/py_env-python3.12/lib/python3.12/site-packages/yapf_third_party/_ylib2to3/pgen2/grammar.py", line 128, in load
    d = pickle.load(f)
        ^^^^^^^^^^^^^^
EOFError: Ran out of input