Add cohort and observation metadata for Churn use case.

Churn use cases include Cohort and Observation use cases. Define the
metadata in the import request for each churn use case.

BUG=chromium:1405080

Change-Id: I872df68847d6d020c1cd7855c67b3d9c123fd049
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4149822
Reviewed-by: Hirthanan Subenderan <hirthanan@google.com>
Commit-Queue: Eric Wang <qianwan@google.com>
Cr-Commit-Position: refs/heads/main@{#1096309}