network-gossip: add metric for number of local messages (#7871)

* network-gossip: add metric for number of local messages

* grandpa: fix GossipEngine missing metrics registry parameter

* network-gossip: increase known messages cache size

* network-gossip: fix tests

* grandpa: remove unnecessary clone

Co-authored-by: Max Inden <mail@max-inden.de>

* network-gossip: count registered and expired messages separately

* network-gossip: add comment on known messages cache size

* network-gossip: extend comment with cache size in memory

Co-authored-by: Max Inden <mail@max-inden.de>