modules (monitoring): Add scrape config for nextcloud-cron

This unit is currently logging JSON, which is hard to read. This adds
promtail pipeline stages to extract JSON fields into labels and replace
the log line with the message field.

modules/nixos/monitoring/default.nix

@@ -52,6 +52,53 @@ in {
                 target_label = "unit";
               }
             ];
+            pipeline_stages = [
+              {
+                match = {
+                  selector = "{unit=\"nextcloud-cron.service\"}";
+                  stages = [
+                    {
+                      json = {
+                        expressions = {
+                          level = "level";
+                          timestamp = "time";
+                          message = "message";
+                          request_id = "reqId";
+                          remote_address = "remoteAddr";
+                          user = "user";
+                          app = "app";
+                          method = "method";
+                          url = "url";
+                          user_agent = "userAgent";
+                          version = "version";
+                          data = "data";
+                        };
+                      };
+                    }
+                    {
+                      labels = {
+                        level = "";
+                        timestamp = "";
+                        request_id = "";
+                        remote_address = "";
+                        user = "";
+                        app = "";
+                        method = "";
+                        url = "";
+                        user_agent = "";
+                        version = "";
+                        data = "";
+                      };
+                    }
+                    {
+                      output = {
+                        source = "message";
+                      };
+                    }
+                  ];
+                };
+              }
+            ];
           }
         ];
       };