stability: make debug tools useful for wedged/underreplicated clusters

[tbg]

For a cluster whose kv store is not available, we can still return useful information. For example,

./cockroach node ls --insecure

could still return a list. Gossip has a lot of information.

It's easy to iterate on these things: stand up a 3x cluster, kill two nodes, see which debug tools break and think about which information they still should be able to return.

More seriously, having locked down the vmodule endpoint with a cluster setting means we're now unable to change it on a wedged cluster, something that @nvanbenschoten and I are presently unhappy about.

[a-robinson]

+1 to this.

Related issues:
#16489
#15342

Gossip has a lot of information.

We do have ./cockroach debug gossip-values, in case you weren't aware.

[tbg]

I think these other issues are good enough, thanks!