Add pullers for debugging.

These pullers return deterministic values so we'll be able to confirm
to validate the data easily server side locally.

For instance,
- the diff of the elapsed_clock_millis field should always be equal to the bucket size
- the diff of pull_count should always be one unless we hit the cache

Test: manual
Change-Id: I4fab30df287e90f8691740d90d87b67022b9c420