*: Make token limit into an instance variable

[subains]

What problem does this PR solve?

Issue Number: ref #33769

Problem Summary:

The option token-limit has historically been a config option. But based on requirements from cloud & PM it should instead be a sysvar.

What is changed and how it works?

This does the minimum work to convert token-limit to a global (cluster-wide) sysvar, and removes the config option (placing it in the previously deprecated list).

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

The TiDB configuration file option `token-limit` has been replaced by the system variable `tidb_token_limit`. This change makes it easier to modify this setting on a cluster wide basis.

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Thank you, and welcome to pingcap/tidb. 😃

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[morgo]

This will be overwritten, but just to save duplication better to use the const for 1000 here:

Suggested change

	TokenLimit = atomic.NewInt32(1000)
	TokenLimit = atomic.NewInt32(DefTiDBTokenLimit)

[morgo]

Some linters complain about uncaught errors. Because the validation is run based on "TypeInt" before this code is called, it will always be a valid number. So we can use our own helper function which doesn't return errors:

Suggested change

	ival, _ := strconv.Atoi(val)
	TokenLimit.Store((int32)(ival))
	return nil
	ival := tidbOptPositiveInt32(val, DefTiDBTokenLimit)
	TokenLimit.Store(int32(ival))
	return nil

[morgo]

There should be a line in config.go for deprecated variables. If you add "token-limit" to it, it will mean a cleaner error for users.

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@subains: PR needs rebase.

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[morgo]

As discussed on Slack - we need to discuss this one more, because it probably should be renamed and not global scope. So I am going to close this PR for now.

Thank you for your contribution! Sorry, we could not merge it this time.