variables: add instance variables

[xhebox]

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• Other reference link(s): Instance Scope Variable tidb#63275

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[Oreoxmt]

Should we add INSTANCE to the scope of tidb_instance_plan_cache_max_size and tidb_stats_cache_mem_quota?

[xhebox]

I checked again, yes, tidb_stats_cache_mem_quota is missing INSTANCE scope. But not tidb_instance_plan_cache_max_size, which has not been moved to instance scope.

[xhebox]

comments LGTM