doc/prometheus.md: Explain the filtering options
doc/prometheus.md: Explain the filtering options

This patch adds a description how to filter by name and labels and how
to remove the help line from the prometheus output.

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

scylladb/seastar,scylladb/seastar,scylladb/seastar