replace info cluster info in a dedicated new section (valkey-io#2964)

In valkey-io#2876 we added cluster info
stats to the info command. However these statistics where added to the
"cluster" section which is part of the default info sections. Since the
calculation of the cluster info is done "by-demand" it can lead to high
server side CPU consumption when many clients are using the "info"
command on a large cluster.

In this PR we separated the cluster info to a dedicated "cluster_stats"
section, which is not enabled by default.

Alternatives considered:

1. Revert valkey-io#2876 - this is
possible as the information is available via `CLUSTER INFO` command. The
only downside is that some "control plane" adaptations need to be placed
in order to gather this information in addition to the "already
collected" server wide `INFO` data.
2. Optimize cluster info status collection - We could dynamically update
and maintain these statistics during cluster topology state changes. The
main "expensive" statistics which are needed to be collected are:
slots_assigned, slots_ok, slots_pfail, slots_fail, nodes_pfail,
nodes_fail, voting_nodes_pfail, voting_nodes_fail
We could track and constantly update these during cluster pings. The
main downside is potential bugs and bad accounting which might be caused
by additional complexity and unhandled edge cases.

---------

Signed-off-by: Ran Shidlansik <ranshid@amazon.com>
Co-authored-by: Harkrishn Patro <bunty.hari@gmail.com>

Author: ranshid

src/server.c

@@ -6431,6 +6431,12 @@ sds genValkeyInfoString(dict *section_dict, int all_sections, int everything) {
                             "# Cluster\r\n"
                             "cluster_enabled:%d\r\n",
                             server.cluster_enabled);
+    }
+
+    /* Cluster Info */
+    if (all_sections || (dictFind(section_dict, "cluster_info") != NULL)) {
+        if (sections++) info = sdscat(info, "\r\n");
+        info = sdscatprintf(info, "# Cluster Info\r\n");
         if (server.cluster_enabled) info = genClusterInfoString(info);
     }