Prevent re-definition of prometheus metrics (#210)

* Prevent re-definition of prometheus metrics

jupyter_server/prometheus/metrics.py

@@ -5,23 +5,29 @@
 conventions for metrics & labels.
 """
 
+try:
+    # Jupyter Notebook also defines these metrics.  Re-defining them results in a ValueError.
+    # Try to de-duplicate by using the ones in Notebook if available.
+    # See https://github.com/jupyter/jupyter_server/issues/209
+    from notebook.prometheus.metrics import HTTP_REQUEST_DURATION_SECONDS, TERMINAL_CURRENTLY_RUNNING_TOTAL, KERNEL_CURRENTLY_RUNNING_TOTAL
 
-from prometheus_client import Histogram, Gauge
+except ImportError:
 
+    from prometheus_client import Histogram, Gauge
 
-HTTP_REQUEST_DURATION_SECONDS = Histogram(
-    'http_request_duration_seconds',
-    'duration in seconds for all HTTP requests',
-    ['method', 'handler', 'status_code'],
-)
+    HTTP_REQUEST_DURATION_SECONDS = Histogram(
+        'http_request_duration_seconds',
+        'duration in seconds for all HTTP requests',
+        ['method', 'handler', 'status_code'],
+    )
 
-TERMINAL_CURRENTLY_RUNNING_TOTAL = Gauge(
-    'terminal_currently_running_total',
-    'counter for how many terminals are running',
-)
+    TERMINAL_CURRENTLY_RUNNING_TOTAL = Gauge(
+        'terminal_currently_running_total',
+        'counter for how many terminals are running',
+    )
 
-KERNEL_CURRENTLY_RUNNING_TOTAL = Gauge(
-    'kernel_currently_running_total',
-    'counter for how many kernels are running labeled by type',
-    ['type']
-)
+    KERNEL_CURRENTLY_RUNNING_TOTAL = Gauge(
+        'kernel_currently_running_total',
+        'counter for how many kernels are running labeled by type',
+        ['type']
+    )