update sample config (was missing cluster-tracing as well).

etc/config.sample.toml

@@ -33,6 +33,12 @@ reporting-disabled = false
   heartbeat-timeout = "1s"
   leader-lease-timeout = "500ms"
   commit-timeout = "50ms"
+  cluster-tracing = false
+
+  # if enabled, when a raft cluster loses a peer due to a `DROP SERVER` command,
+  # the leader will automatically ask a non-raft peer node to promote to a raft
+  # peer.  This only happens if there is a non-raft peer node available to promote.
+  raft-promotion-enabled = true
 
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 ### [data]