refactor(logging): move the console line format into src/logging/format.ts

The printf template of config/logger.ts becomes formatLogLine, with the same
logic, so it can be unit tested without winston.

No behaviour change.

Refs libretech/mp#71

config/logger.ts

@@ -1,16 +1,10 @@
 import { winston } from "@strapi/logger";
+import { formatLogLine } from "../src/logging/format";
 
 export default {
 	transports: [
 		new winston.transports.Console({
-			format: winston.format.combine(
+			format: winston.format.combine(winston.format.printf(formatLogLine))
-				winston.format.printf(({ timestamp, level, message, ...rest }) => {
-					let restString = JSON.stringify(rest, undefined, 2);
-					restString = restString === "{}" ? "" : restString;
-
-					return `${timestamp}Z ${level}: ${message} ${restString}`;
-				})
-			)
 		})
 	]
 };

src/logging/format.ts

@@ -0,0 +1,14 @@
+/** What winston hands a printf template (logform's TransformableInfo); Strapi's default format has added `timestamp`. */
+export type LogInfo = {
+	level: string;
+	message: unknown;
+	[key: string | symbol]: unknown;
+};
+
+/** One console line: `<timestamp>Z <level>: <message> <all other fields as indented JSON, if any>`. */
+export const formatLogLine = ({ timestamp, level, message, ...rest }: LogInfo): string => {
+	let restString = JSON.stringify(rest, undefined, 2);
+	restString = restString === "{}" ? "" : restString;
+
+	return `${timestamp}Z ${level}: ${message} ${restString}`;
+};