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
