metrics: alternative metrics creation with labels
metrics: alternative metrics creation with labels

The added metrics creation functions support both labels and putting the
lambda function as the last parameter.

Signed-off-by: Amnon Heiman <ed5fc74c4692051ad0b19749fd7cea6ec2d74b9e@scylladb.com>
Message-Id: <1486460750-590-1-git-send-email-ed5fc74c4692051ad0b19749fd7cea6ec2d74b9e@scylladb.com>

dreamsxin/seastar,cloudius-systems/seastar,avikivity/seastar,syuu1228/seastar,cloudius-systems/seastar,scylladb/seastar,scylladb/seastar,avikivity/seastar,avikivity/seastar,syuu1228/seastar,cloudius-systems/seastar,scylladb/seastar,dreamsxin/seastar,dreamsxin/seastar,syuu1228/seastar