refactor (monitoring): remove large try block anti-pattern

change the overly large try block into a small-factor guard to increase readability though indent reduction

prometheus.py

@@ -12,25 +12,28 @@ EXCLUDED_USER_AGENTS = ('bot', 'spider', 'crawler', 'monitor', 'curl',
 def start_monitor(app: FastAPI) -> None:
     try:
         import prometheus_client
-        visits = prometheus_client.Counter('nicegui_page_visits', 'Number of real page visits',
-                                           ['path', 'session', 'origin'])
-
-        class PrometheusMiddleware(BaseHTTPMiddleware):
-            async def dispatch(self, request: Request, call_next):
-                if 'id' not in request.session:
-                    request.session['id'] = str(uuid.uuid4())
-                response = await call_next(request)
-                if response.headers.get('x-nicegui-content') == 'page':
-                    agent = request.headers.get('user-agent', 'unknown').lower()
-                    # ignore monitoring, web crawlers and the like
-                    if not any(s in agent for s in EXCLUDED_USER_AGENTS):
-                        origin_url = request.headers.get('referer', 'unknown')
-                        visits.labels(request.get('path'), request.session['id'], origin_url).inc()
-                return response
-
-        if inspect.stack()[-2].filename.endswith('spawn.py'):
-            prometheus_client.start_http_server(9062)
-
-        app.add_middleware(PrometheusMiddleware)
     except ModuleNotFoundError:
         logging.info('Prometheus not installed, skipping monitoring')
+        return
+      
+    visits = prometheus_client.Counter('nicegui_page_visits', 'Number of real page visits',
+                                       ['path', 'session', 'origin'])
+
+    class PrometheusMiddleware(BaseHTTPMiddleware):
+        async def dispatch(self, request: Request, call_next):
+            if 'id' not in request.session:
+                request.session['id'] = str(uuid.uuid4())
+            response = await call_next(request)
+            if response.headers.get('x-nicegui-content') == 'page':
+                agent = request.headers.get('user-agent', 'unknown').lower()
+                # ignore monitoring, web crawlers and the like
+                if not any(s in agent for s in EXCLUDED_USER_AGENTS):
+                    origin_url = request.headers.get('referer', 'unknown')
+                    visits.labels(request.get('path'), request.session['id'], origin_url).inc()
+            return response
+
+    if inspect.stack()[-2].filename.endswith('spawn.py'):
+        prometheus_client.start_http_server(9062)
+
+    app.add_middleware(PrometheusMiddleware)
+