Tornado integration raises `KeyError` when xsrf check fails

Describe your environment

• Python 3.10
• opentelemetry-instrumentation-tornado 0.41b0
• tornado 6.1.0

Steps to reproduce

• Create a Tornado app with xsrf_cookies enabled
• Make a POST request to an endpoint without a xsrf header
• Check server logs to see KeyError exception being logged

What is the expected behavior?
Tornado warns that there is a missing xsrf cookie but does not raise an execption.

What is the actual behavior?

2023-10-13 13:48:02,233:web.py:1757:MainThread:ERROR: - Error in exception logger
Traceback (most recent call last):
  File "/tmp/plz_sandbox/tornado_test.pex/third_party/python3/tornado/web.py", line 1679, in _execute
    self.check_xsrf_cookie()
  File "/tmp/plz_sandbox/tornado_test.pex/third_party/python3/tornado/web.py", line 1519, in check_xsrf_cookie
    raise HTTPError(403, "'_xsrf' argument missing from POST")
tornado.web.HTTPError: HTTP 403: Forbidden ('_xsrf' argument missing from POST)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/tmp/plz_sandbox/tornado_test.pex/third_party/python3/tornado/web.py", line 1753, in _handle_request_exception
    self.log_exception(*sys.exc_info())
  File "/tmp/plz_sandbox/tornado_test.pex/third_party/python3/opentelemetry/instrumentation/tornado/__init__.py", line 400, in _log_exception
    _record_on_finish_metrics(server_histograms, handler, error)
  File "/tmp/plz_sandbox/tornado_test.pex/third_party/python3/opentelemetry/instrumentation/tornado/__init__.py", line 586, in _record_on_finish_metrics
    (default_timer() - server_histograms[_START_TIME]) * 1000
KeyError: 'start_time'

Additional context
The issues seems to be that Tornado does the xsrf checking before the prepare statement is run so the opentelemetry integration doesn't have the chance to initialise the _START_TIME value: https://github.com/open-telemetry/opentelemetry-python-contrib/blob/v1.16.0/instrumentation/opentelemetry-instrumentation-tornado/src/opentelemetry/instrumentation/tornado/__init__.py#L371

As far as I can tell this issue still exists in the latest versions of Tornado and this library.

Read instrumentation/opentelemetry-instrumentation-tornado/src/opentelemetry/instrumentation/tornado/init.py, especially _log_exception and _record_on_finish_metrics near the referenced line. Reproduce a POST without an xsrf header and verify that the failed check logs Tornado's warning without a KeyError for the missing start_time value.