trace2: log progress time and throughput

trace2: log progress time and throughput

Rather than teaching only one operation, like 'git fetch', how to write
down throughput to traces, we can learn about a wide range of user
operations that may seem slow by adding tooling to the progress library
itself. Operations which display progress are likely to be slow-running
and the kind of thing we want to monitor for performance anyways. By
showing object counts and data transfer size, we should be able to
make some derived measurements to ensure operations are scaling the way
we expect.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>