histogram: Add started counter
histogram: Add started counter

This patch adds a started counter, that is used to mark the number of
operation that were started.

This counter serves two purposes, it is a better indication for when to
sample the data and it is used to indicate how many pending operations
are.

Signed-off-by: Amnon Heiman <ed5fc74c4692051ad0b19749fd7cea6ec2d74b9e@scylladb.com>

kjniemi/scylla,duarten/scylla,raphaelsc/scylla,scylladb/scylla,duarten/scylla,kjniemi/scylla,raphaelsc/scylla,kjniemi/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,raphaelsc/scylla