Add metric to count unapproved parachain blocks contained in unfinalized chain.

[sandreim]

Right now we only have logs: 2023-04-24 12:21:24.830 DEBUG tokio-runtime-worker parachain::approval-voting: approved blocks 15534167-[11111111 1111111111 1111111111 1111111111 1111111111 1111111111 1111111111 1111111111 1111111111 1111111111 1111111111 1111111111 1111111111 1111111111 1111111111 1111111111 1111111111 1111111111 1111111111 1111111111 11]-15533968... (truncated due to large window) but these are limited by a 200 sized tracing window.

[bkchr]

While doing this, someone could also just print out what are the unapproved parachain blocks instead of printing a lot of ones :D