Add prometheus info metrics listing server extensions + versions

Follow-up to jupyter-server#1467.

Author: yuvipanda

Diff for: jupyter_server/prometheus/metrics.py

@@ -53,6 +53,7 @@
 
 # New prometheus metrics that do not exist in notebook v6 go here
 SERVER_INFO = Info("jupyter_server", "Jupyter Server Version information")
+SERVER_EXTENSION_INFO = Info("jupyter_server_extension", "Jupyter Server Extensiom Version Information", ["extension_name", "package_name", "version", "enabled"])
 
 __all__ = [
     "HTTP_REQUEST_DURATION_SECONDS",

Diff for: jupyter_server/serverapp.py

@@ -110,7 +110,7 @@
     GatewaySessionManager,
 )
 from jupyter_server.log import log_request
-from jupyter_server.prometheus.metrics import SERVER_INFO
+from jupyter_server.prometheus.metrics import SERVER_INFO, SERVER_EXTENSION_INFO
 from jupyter_server.services.config import ConfigManager
 from jupyter_server.services.contents.filemanager import (
     AsyncFileContentsManager,
@@ -2703,6 +2703,14 @@ def init_metrics(self) -> None:
         """
         SERVER_INFO.info({"version": __version__})
 
+        for k, ext in self.extension_manager.extensions.items():
+            SERVER_EXTENSION_INFO.labels(
+                package_name=k,
+                extension_name=ext.name,
+                version=ext.version,
+                enabled=str(ext.enabled).lower()
+            )
+
     @catch_config_error
     def initialize(
         self,