[zircon][cobalt-client] Provide InMemoryLogger.

Instead of virtualizing the Collector, which only sees a metric as a
flushable and requires less legible ways for verification, provide a
test-only InMemoryLogger class which exposes the necesary internal types
within its class namespace and keeps tracks of the logged data.

TEST=cobalt-client-test, cobalt-client-unit-test

Change-Id: Ib67cb90f3b974a61ebe5fada424dd4b4a27cb1cb