Support `setAuthenticatedUserContext`

The ApplicationInsights-JS SDK supports a method called setAuthenticatedUserContext which sends a custom auth token with each event.

The client code is basically just doing some validation then updating the ai.user.authUserId tag.

On the server, it looks like the same thing can be done, but is not documented:

let client = appInsights.defaultClient;
let key = client.context.keys.userAuthUserId;
client.context.tags[key] = <authToken>;

Providing this same method on the server side would be helpful for several reasons:

• A users client and server activity can be tied together by the custom auth id
• SDK users don’t have to figure this out themselves
• SDK consistency

[glenjamin]

This is what I ended up going with

appInsights.defaultClient.addTelemetryProcessor(attachUserId);
function attachUserId(envelope, context) {
  const res = context['http.ServerResponse'];
  if (res && res.locals && res.locals.user) {
    envelope.tags['ai.user.authUserId'] = res.locals.user.username;
  }
}

[OsvaldoRosado]

Great catch!

The JS SDK can also provide this userId to the server-side using the “ai_authUser” cookie. It’s my understanding that our .NET SDK will read this cookie and automatically set the authUserId that it received from the JS SDK. It would be nice to implement that here as well.