feat(RequestLogging): Include err field (#56)

Consuming applications tend towards the following `logFn` boilerplate:

```typescript
const data = {
  ...fields,
  err: err ?? ErrorMiddleware.thrown(ctx),
};
```

While Koala provides a simplified `internalErrorString`, rich error
details are often handy for troubleshooting, and the `err` property name
is blessed by common logging frameworks like Bunyan and Pino. This
preserves `internalErrorString` for backward compatibility.

src/requestLogging/README.md

@@ -153,6 +153,11 @@ const healthCheckHandler = async (ctx: Koa.Context) => {
   },
   "status": 500,
   "internalErrorString": "ExpiredTokenException: The security token included in the request is expired",
+  "err": {
+    "message": "The security token included in the request is expired",
+    "name": "ExpiredTokenException",
+    "stack": "..."
+  },
   "msg": "request log",
   "time": "2018-10-16T00:44:41.055Z",
   "v": 0

src/requestLogging/requestLogging.test.ts

@@ -197,6 +197,7 @@ describe('RequestLogging', () => {
         },
         `
         Object {
+          "err": [Error: Something tragic happened],
           "headers": Object {
             "accept": undefined,
             "accept-encoding": "gzip, deflate",

src/requestLogging/requestLogging.ts

@@ -1,5 +1,6 @@
 import Koa from 'koa';
 
+import { thrown } from '../errorMiddleware/errorMiddleware';
 import { tracingFromContext } from '../tracingHeaders/tracingHeaders';
 
 /**
@@ -126,7 +127,7 @@ export const createMiddleware = <StateT extends State, CustomT>(
 
     const requestFinished = (
       resultFields: Record<string, unknown>,
-      error?: unknown,
+      err?: unknown,
     ) => {
       if (ctx.state.skipRequestLogging) {
         return;
@@ -136,20 +137,25 @@ export const createMiddleware = <StateT extends State, CustomT>(
       logFn(
         ctx,
         {
+          ...(typeof err !== 'undefined' && {
+            err,
+            internalErrorString: String(err),
+          }),
           latency,
           headers: replaceHeaders(ctx.request.header, headerReplacements),
           ...contextFields(ctx),
           ...resultFields,
         },
-        error,
+        err,
       );
     };
 
     try {
       await next();
-      requestFinished({ status: ctx.response.status });
+
+      requestFinished({ status: ctx.response.status }, thrown(ctx));
     } catch (err: unknown) {
-      requestFinished({ status: 500, internalErrorString: String(err) }, err);
+      requestFinished({ status: 500 }, err);
 
       throw err;
     }