main: stop prometheus server when shutting down
main: stop prometheus server when shutting down

This patch adds a enging().on_exit cleanup for the prometheus server,
similar to other components in the system.

It will stop the server when sutting down.

Fixes #2520
Message-Id: <721dddbd764a664047ed574a9a6f1d07a2526fd2@scylladb.com>

scylladb/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,duarten/scylla,duarten/scylla