Prometheus Metrics Duplication Error

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As it says on the tin, re-defining these metrics results in an error. This bug is preventing the notebook and jupyter_server packages from peacefully co-existing.

In [3]: from notebook import log

In [4]: from jupyter_server import log
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-4-026e191fd69c> in <module>
----> 1 from jupyter_server import log

/tmp/conda_envs/0d2c69b0eca52b083fd9c7e9/lib/python3.8/site-packages/jupyter_server/log.py in <module>
      8 import json
      9 from tornado.log import access_log
---> 10 from .prometheus.log_functions import prometheus_log_method
     11
     12

/tmp/conda_envs/0d2c69b0eca52b083fd9c7e9/lib/python3.8/site-packages/jupyter_server/prometheus/log_functions.py in <module>
----> 1 from .metrics import HTTP_REQUEST_DURATION_SECONDS
      2
      3
      4 def prometheus_log_method(handler):
      5     """

/tmp/conda_envs/0d2c69b0eca52b083fd9c7e9/lib/python3.8/site-packages/jupyter_server/prometheus/metrics.py in <module>
     10
     11
---> 12 HTTP_REQUEST_DURATION_SECONDS = Histogram(
     13     'http_request_duration_seconds',
     14     'duration in seconds for all HTTP requests',

/tmp/conda_envs/0d2c69b0eca52b083fd9c7e9/lib/python3.8/site-packages/prometheus_client/metrics.py in __init__(self, name, documentation, labelnames, namespace, subsystem, unit, registry, labelvalues, buckets)
    489                  ):
    490         self._prepare_buckets(buckets)
--> 491         super(Histogram, self).__init__(
    492             name=name,
    493             documentation=documentation,

/tmp/conda_envs/0d2c69b0eca52b083fd9c7e9/lib/python3.8/site-packages/prometheus_client/metrics.py in __init__(self, name, documentation, labelnames, namespace, subsystem, unit, registry, labelvalues)
    105             # Register the multi-wrapper parent metric, or if a label-less metric, the whole shebang.
    106             if registry:
--> 107                 registry.register(self)
    108
    109     def labels(self, *labelvalues, **labelkwargs):

/tmp/conda_envs/0d2c69b0eca52b083fd9c7e9/lib/python3.8/site-packages/prometheus_client/registry.py in register(self, collector)
     25             duplicates = set(self._names_to_collectors).intersection(names)
     26             if duplicates:
---> 27                 raise ValueError(
     28                     'Duplicated timeseries in CollectorRegistry: {0}'.format(
     29                         duplicates))

ValueError: Duplicated timeseries in CollectorRegistry: {'http_request_duration_seconds_sum', 'http_request_duration_seconds_created', 'http_request_duration_seconds_bucket', 'http_request_duration_seconds_count'}

[ivanov]

There can be only one!

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