This PR extends the metrics exposed by the peerstore with the total
number of banned peers.

The new metric is exposed under
`substrate_sub_libp2p_peerset_num_banned_peers`.

To easily extend metrics in the future, the `fn num_known_peers` is
removed in favor of `fn status`.

While at it, enable the metrics for litep2p:
- total number of peers from peerstore (needed to debug memory
consumption)
- total number of banned peers from peerstore (needed to debug
reputation bans and disconnects)

Have added a couple of tests to validate that the number of banned peers
is exposed properly.

Part of: https://github.com/paritytech/polkadot-sdk/issues/4681


### Testing Done
Using [subp2p-explorer](https://github.com/lexnv/subp2p-explorer) have
submitted random data on tx protocol.
The peer gets banned, the num of banned peers is incremented then the
peer is disconnected.

cc @paritytech/networking

---------

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>
Co-authored-by: Dmitry Markin <dmitry@markin.tech>