Cluster page diagnostic improvements

[KodrAus]

Seq's disaster recovery is a quiet background process in the steady state, but requires a lot of moving parts to initially set up. It requires configuration on Seq, its host, and possibly in cloud networks. To assist the setup process, we've exposed more of the cluster metrics and diagnostics on the cluster page itself.

If you set up a cluster and don't see any of its metrics increment, then you know the follower node isn't reaching the leader. If the authentication key is invalid then you'll see that too.

The diagnostic log follows the same approach as the ingestion log. It only includes errors, but if there's some critical failure in the process it'll be surfaced there too.