(core) Add optional telemetry to grist-core

Summary:
Adds support for optional telemetry to grist-core.

A new environment variable, GRIST_TELEMETRY_LEVEL, controls the level of telemetry collected.

Test Plan: Server and unit tests.

Reviewers: paulfitz

Reviewed By: paulfitz

Subscribers: dsagal, anaisconce

Differential Revision: https://phab.getgrist.com/D3880

app/client/ui/OpenVideoTour.ts

@@ -37,7 +37,7 @@ const VIDEO_TOUR_YOUTUBE_EMBED_ID = 'qnr2Pfnxdlc';
         if (youtubePlayer.isLoading()) { return; }
 
         logTelemetryEvent('watchedVideoTour', {
-          watchTimeSeconds: Math.floor(youtubePlayer.getCurrentTime()),
+          limited: {watchTimeSeconds: Math.floor(youtubePlayer.getCurrentTime())},
         });
       });