Make token-limit configurable as an INSTANCE scoped variable

[morgo]

Enhancement

This is forked from #33769

For token-limit there is a use-case to make it online configurable, but it's not a great fit for GLOBAL. This enhancement proposes that we add an instance scoped variable for it.

For the name, I suggest tidb_max_connection_concurrency or tidb_connection_concurrency_limit.

This is different from the issues in #33769 because it is not a breaking change. It can still be configured in the config file as token-limit or via the instance scoped variable that we are introducing.

[morgo]

@CbcWestwolf would you like to take a look at this?

[CbcWestwolf]

@CbcWestwolf would you like to take a look at this?

Sure! Let me do this convertion.

[CbcWestwolf]

/assign

[CbcWestwolf]

It can still be configured in the config file as token-limit or via the instance scoped variable that we are introducing.

Should we move the token-limit to [instance] section in config file just like what we have done in #33279?

[morgo]

It can still be configured in the config file as token-limit or via the instance scoped variable that we are introducing.

Should we move the token-limit to [instance] section in config file just like what we have done in #33279?

Yes. Under [instance] it should have the same name as the sysvar. For the sysvar name, I suggested tidb_max_connection_concurrency or tidb_connection_concurrency_limit. The problem with "token limit" is it is too tied to an implementation, not what the feature does (a cap on concurrent connections that can be executing).