[NEW] Introduce server level network ingress / egress metrics for cluster bus.

[kyle-yh-kim]

The problem/use-case that the feature addresses Valkey server already accumulates the following networking server statistics;

// server.h
long long stat_net_input_bytes;                /* Bytes read from network. */
long long stat_net_output_bytes;               /* Bytes written to network. */
long long stat_net_repl_input_bytes; /* Bytes read during replication, added to stat_net_input_bytes in 'info'. */
/* Bytes written during replication, added to stat_net_output_bytes in 'info'. */
long long stat_net_repl_output_bytes;

I propose the following statistics for all cluster bus operations. Primary hook-point could be upon processClusterPacket().

long long stat_net_clusterbus_input_bytes;                /* Bytes read from network. */
long long stat_net_clusterbus_output_bytes;               /* Bytes written to network. */

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[zuiderkwast]

Do you want to show them in INFO, under network or under cluster? What to show for non-cluster mode? (Zero or omit the fields?)

Shall we store these fields under server->cluster so they only exist in cluster mode?