kvserver: warn on slow raft schedule

**Is your feature request related to a problem? Please describe.**

We saw in a recent escalation[^1] unexplained heartbeat latencies. It took a
good amount of time to figure out that the `raft.scheduler.latency` p99s were
at ~1s on the liveness leaseholder.

Similar to this warning

https://github.com/cockroachdb/cockroach/blob/736a67e0d36cc545bf74d65db069ee895ff9bea0/pkg/kv/kvserver/store_raft.go#L653-L655

we could have a (perhaps rate-limited) warning whenever it takes "long" to
schedule a raft group, as this will translate to elevated latencies in both
internal and client-facing operations.

[^1]: https://github.com/cockroachlabs/support/issues/2107

Jira issue: CRDB-24959