prometheus: to_str(metric_value): convert also ABSOLUTE
prometheus: to_str(metric_value): convert also ABSOLUTE

Convert data_type::ABSOLUTE same as data_type::COUNTER.

Switch all cases now to let the compiler warn
if not all cases were handled explicitly.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

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