assume a generous prior in the rate estimator

currently with one data point the estimator is biased high. If we don't
have any data we want to be biased low to allow the first interaction
to be an isolated burst.

Also add metrics for enqueue rate to make it easier to debug this
code in the future (currently we only have metrics after the
rate limiter).

Bug: 29379789
Change-Id: I7b1d379290a55f120d039fd29d1bc0abbc6d4932