Collecting Request:Count metrics

This sets up the collection of the request:count metrics, with unique,
unknown request strings, cut off at a certain point (20 characters to
the end). This is in line with our designed plans, but the exact cut off
point may be discussed. The idea is that the server side context is all
that's needed for queries.

Bug: 271135048
Test: Build + E2E Test in progress (won't submit till that is done)
Change-Id: I5420f6dd09f5b52f71926ad20d89d82f9b369b24