Bug 2055331 - Report reason in unenrollment telemetry

[freshstrangemusic]

The nimbsus_events.unenrollment metric is supposed to contain the reason field, but it just hasn't been reported.

Additionally, this removes "is_rollout" field from the nimbus_events.enrollment metric as (a) that data is not already reported, (b) the data is not already available on the EnrollmentChangeEvent (and adding it would be a breaking change), and (c) the field is not all that useful.

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[freshstrangemusic]

Data review is not necessary since this is only metric field removal