fix(metrics): fix issue where package download counters were not full…

…y accurate

During real world testing, the count of package downloads almost always came in lower then it should
have. It seems that sometimes the npm client kills the connection prior to Express thinking the full
response has been sent. The solution seems to be to rely on the `close` event instead of the
`finish` event on the response object.

fix #7

src/metrics.ts

@@ -91,7 +91,7 @@ export default class VerdaccioMiddlewarePlugin implements IPluginMiddleware<Metr
 
     // We won't know the final status code until the response is sent to the client. Because of this we don't collect
     // the metrics for this request until the response 'finish' event is emitted.
-    res.once('finish', () => {
+    res.once('close', () => {
       const { statusCode } = res;
       const metricLabels: MetricsLabels = { username, userAgentName, statusCode };
       if (packageGroup) {

tests/testUtils.ts

@@ -95,7 +95,7 @@ export function generateMockResponse() {
     status: jest.fn(),
     send: jest.fn(),
     once: jest.fn().mockImplementation(function (eventName, handler: any) {
-      if (eventName === 'finish') {
+      if (eventName === 'close') {
         mockResponse.statusCode = 200;
         mockResponse.finish.mockImplementation(handler);
       }