Prefix PostHog identify id with "xpro:" for global uniqueness

[ChristopherChudzicki]

What are the relevant tickets?

Part of https://github.com/mitodl/hq/issues/12601 (step 4)

Description (What does it do?)

Changes how xPro identifies posthog users.

• Previously: identify(pk), like pk=123 user id primary key.
  • We use the same posthog project in mit-learn and xpro, so until now, whether Posthog User 123 was xpro or learn user 123 was entirely dependent on where they went first.
• Now: Prefixed with xpro:pk

Ideally, this would be the same guid used in Learn and MITxOnline an (maybe, one day, ocw). But xpro isn't hooked up to keycloak, so we can't get the guid.

How can this be tested?

No functional UI change, so this is best verified by manual code review:

• static/js/components/Header.js: confirm identify is now called with `xpro:${currentUser.id}` instead of the bare id.
• Unit test added: Header_test.js.

Additional Context

Companion MITxOnline PR for step 3 of the same issue: mitodl/mitxonline#3798

[Anas12091101]

Heads up, this isn't a bug, just something to be aware of.

Right now PostHog knows each xPro user by their plain ID (e.g. 123). After this change, the same person becomes xpro:123. PostHog will treat that as a completely new person it won't connect their old activity to their new activity

[ChristopherChudzicki]

Right now PostHog knows each xPro user by their plain ID (e.g. 123). After this change, the same person becomes xpro:123. PostHog will treat that as a completely new person it won't connect their old activity to their new activity

This is unfortunately not quite true 😄 Posthog user 123 is one of these two people:

• Learn user with primary key 123
• OR xpro user with primary key 123

Both apps used identify(pk) within the same posthog project.

Anyway, that's the reason for this change. Disambiguate the two.

Learn and MITxOnline are both moving to GUIDs, xPro doesn't have access to the GUID, though.