Add tracing fields to spec (#29)

Author: felixbarny

spec/README.md

@@ -60,6 +60,8 @@ The following example describes a richer set of fields in an event that has not
         "app_uptime": "2016-05-15T14:03:12.345Z"
     },
     "tags": ["production", "env001"],
+    "trace.id": "4bf92f3577b34da6a3ce929d0e0e4736",
+    "transaction.id": "00f067aa0ba902b7"
 }
 ```
 

spec/spec.json

@@ -38,6 +38,18 @@
                     "substitute": "_"
                 }
             }
+        },
+        "trace.id": {
+            "type": "string",
+            "required": false,
+            "url": "https://www.elastic.co/guide/en/ecs/current/ecs-tracing.html",
+            "comment": "When APM agents add this field to the context, ecs loggers should pick it up and add it to the log event."
+        },
+        "transaction.id": {
+            "type": "string",
+            "required": false,
+            "url": "https://www.elastic.co/guide/en/ecs/current/ecs-tracing.html",
+            "comment": "When APM agents add this field to the context, ecs loggers should pick it up and add it to the log event."
         }
     }
-}
+}