metrics.hh: Support SUMMARY type
metrics.hh: Support SUMMARY type

This patch add support for the summary type on the metrics layer.

A summary is a different kind of histogram, it's buckets are percentile
so the reporting layer (i.e. Prometheus for example) would know to
report it correctly.

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

syuu1228/seastar,scylladb/seastar,avikivity/seastar,scylladb/seastar,avikivity/seastar,syuu1228/seastar,avikivity/seastar,syuu1228/seastar,scylladb/seastar