add metrics endpoint

app.py

@@ -4,7 +4,7 @@ import json
 import httpx
 import configparser
 
-from flask import Flask, request, jsonify, abort
+from flask import Flask, request, jsonify, abort, Response
 from sqlalchemy import create_engine, Column, Integer, String
 from sqlalchemy.orm import declarative_base, sessionmaker
 
@@ -43,7 +43,6 @@ Base.metadata.create_all(engine)
 app = Flask(__name__)
 
 
-
 def get_auth_header():
     auth = request.headers.get("Authorization", "")
     if not auth.startswith("Bearer "):
@@ -173,6 +172,26 @@ def notify():
     })
 
 
+@app.route("/metrics", methods=["GET"])
+def metrics():
+    session = SessionLocal()
+
+    try:
+        token_count = session.query(DeviceToken).count()
+    finally:
+        session.close()
+
+    metrics_output = (
+        "# HELP apns_device_tokens_total Number of registered APNs device tokens\n"
+        "# TYPE apns_device_tokens_total gauge\n"
+        f"apns_device_tokens_total {token_count}\n"
+    )
+
+    return Response(
+        metrics_output,
+        mimetype="text/plain; version=0.0.4"
+    )
+
 
 if __name__ == "__main__":
     app.run(host="0.0.0.0", port=3000, debug=True)