prometheus: Expose 3 activity metrics (#1471)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: yuvipanda

jupyter_server/prometheus/metrics.py

@@ -58,6 +58,17 @@
     "Jupyter Server Extensiom Version Information",
     ["name", "version", "enabled"],
 )
+LAST_ACTIVITY = Gauge(
+    "jupyter_server_last_activity_timestamp_seconds",
+    "Timestamp of last seen activity on this Jupyter Server",
+)
+SERVER_STARTED = Gauge(
+    "jupyter_server_started_timestamp_seconds", "Timestamp of when this Jupyter Server was started"
+)
+ACTIVE_DURATION = Gauge(
+    "jupyter_server_active_duration_seconds",
+    "Number of seconds this Jupyter Server has been active",
+)
 
 __all__ = [
     "HTTP_REQUEST_DURATION_SECONDS",

jupyter_server/serverapp.py

@@ -110,7 +110,13 @@
     GatewaySessionManager,
 )
 from jupyter_server.log import log_request
-from jupyter_server.prometheus.metrics import SERVER_EXTENSION_INFO, SERVER_INFO
+from jupyter_server.prometheus.metrics import (
+    ACTIVE_DURATION,
+    LAST_ACTIVITY,
+    SERVER_EXTENSION_INFO,
+    SERVER_INFO,
+    SERVER_STARTED,
+)
 from jupyter_server.services.config import ConfigManager
 from jupyter_server.services.contents.filemanager import (
     AsyncFileContentsManager,
@@ -2708,6 +2714,16 @@ def init_metrics(self) -> None:
                 name=ext.name, version=ext.version, enabled=str(ext.enabled).lower()
             )
 
+        started = self.web_app.settings["started"]
+        SERVER_STARTED.set(started.timestamp())
+
+        LAST_ACTIVITY.set_function(lambda: self.web_app.last_activity().timestamp())
+        ACTIVE_DURATION.set_function(
+            lambda: (
+                self.web_app.last_activity() - self.web_app.settings["started"]
+            ).total_seconds()
+        )
+
     @catch_config_error
     def initialize(
         self,