Optimize heartbeat payload size

[MichaelScofield]

If a datanode contains over 10,000 regions, i wonder that if the heartbeat request is too large.

Originally posted by @killme2008 in https://github.com/GreptimeTeam/greptimedb/pull/1558#discussion_r1196490874

As the comment above suspects, it's needed to see how the heartbeat payload size is when it contains very many region stats. Optimization should be taken in place to reduced the bandwidth usage (and the cpu time and memory to process them of course).