1829754 - Correct the Metrics doc's explanation of message IDs in about:welcome pings

Status: NEW

(Firefox :: Messaging System, task, P2)

Description

[Shane Hughes [:aminomancer]]

These example pings are no longer accurate, since the format of message_id has changed significantly.

For impression pings, we actually follow this format:

${message_id}_${screen_index}_${screen_id}_${screen_initials}
MR_WELCOME_DEFAULT_0_AW_EASY_SETUP_EPSIMG

And for other pings:

${message_id}_${screen_index}_${screen_id}
MR_WELCOME_DEFAULT_0_AW_EASY_SETUP

So there's not a small list of possible values anymore. It would make more sense, in this documentation, to put a comment explaining how this works and what to expect, with just one example string, instead of an array of pre-MR example strings.