doc/prometheus: note that metric family is passed by query name
doc/prometheus: note that metric family is passed by query name

we started to use query params instead of part of URL since
c02150ef3b49cafe79511447ed48b038b4d0624e, but the document was not
updated accordingly. in this change, the accompanied document is
updated.

Signed-off-by: Kefu Chai <cdc92d111bae0ed1ee3b3c8a1f849fcffc6b44f7@gmail.com>
Message-Id: <20210621073556.90357-1-cdc92d111bae0ed1ee3b3c8a1f849fcffc6b44f7@gmail.com>

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