(core) Fix recording of signups, and record as telemetry too.

Summary:
Previously we failed to log signup info for users who signed up via
Google. This fixes that issue by recording it on first post-signup
visit. It also includes signup as a new telemetry event, recorded at the
same point.

Test Plan: Tested locally to see that a signup produces an appropriate log message and telemetry event.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3921

app/server/lib/FlexServer.ts

@@ -866,6 +866,9 @@ export class FlexServer implements GristServer {
             recordSignUpEvent: true
           }});
 
+          // Give a chance to the login system to react to the first visit after signup.
+          this._loginMiddleware.onFirstVisit?.(req);
+
           const domain = mreq.org ?? null;
           if (!process.env.GRIST_SINGLE_ORG && this._dbManager.isMergedOrg(domain)) {
             // We're logging in for the first time on the merged org; if the user has

app/server/lib/GristServer.ts

@@ -75,6 +75,8 @@ export interface GristLoginMiddleware {
   // or null if anonymous (and other methods of determining profile such
   // as a cookie should not be used), or undefined to use other methods.
   getProfile?(req: express.Request|IncomingMessage): Promise<UserProfile|null|undefined>;
+  // Called on first visit to an app page after a signup, for reporting or telemetry purposes.
+  onFirstVisit?(req: express.Request): void;
 }
 
 /**