feat: add metrics endpoint and improve month handling

Added a new /_metrics/ endpoint to expose financial data in Prometheus format, facilitating better monitoring and analytics.

Enhanced the get_latest_month function to optionally exclude the current month, preventing potential issues in incomplete data scenarios, with this behavior being toggled by the application's development mode status.

main.py

@@ -54,7 +54,9 @@ def catch_all(path):
                 (pathlib.Path(__file__).parent / "data" / "finances.json").read_text()
             )
 
-            finances_month, finances_year = get_latest_month(finances)
+            allow_current = app.development_mode
+
+            finances_month, finances_year = get_latest_month(finances, allow_current)
             finances_period = datetime.date(finances_year, finances_month, 1)
             finances_period_str = finances_period.strftime("%B %Y")
 
@@ -97,6 +99,25 @@ def catch_all(path):
         return "404 Not Found", 404
 
 
+@app.route("/_metrics/")
+def metrics():
+    finances = json.loads(
+        (pathlib.Path(__file__).parent / "data" / "finances.json").read_text()
+    )
+
+    balances = get_transparency_data(finances)["end_balance"]
+
+    response = (
+        "# HELP privatecoffee_balance The balance of the private.coffee account\n"
+    )
+    response += "# TYPE privatecoffee_balance gauge\n"
+
+    for currency, balance in balances.items():
+        response += f'privatecoffee_balance{{currency="{currency}"}} {balance}\n'
+
+    return response
+
+
 app.development_mode = False
 
 if os.environ.get("PRIVATECOFFEE_DEV"):