kvserverbase: add COCKROACH_DISABLE_MMA env var override

[tbg]

Summary

• Add a COCKROACH_DISABLE_MMA environment variable that forces
  kv.allocator.load_based_rebalancing to off, regardless of the cluster
  setting value. This is an emergency kill switch for when MMA causes crashes
  too frequent to change the setting through normal means.
• Unexport the LoadBasedRebalancingMode setting variable so all access goes
  through GetLoadBasedRebalancingMode, preventing callers from bypassing the
  override.

Early commits are mechanical migrations of .Get() and .Override() call
sites; the last commit unexports the setting.

Epic: none

[trunk-io]

😎 Merged successfully - details.

[cockroach-teamcity]

This change is 

[wenyihu6]

mod one comment.

@wenyihu6 made 2 comments.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on tbg).

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pkg/kv/kvserver/kvserverbase/base.go line 143 at r1 (raw file):

func GetLoadBasedRebalancingMode(sv *settings.Values) LBRebalancingMode {
	if disableMMA {
		return LBRebalancingOff

Should we fall back to leases-and-replicas instead - LBRebalancingOff disables old store rebalancer as well. Added a commit for this.

[tbg]

TFTR!

/trunk merge

@tbg made 2 comments.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on wenyihu6).

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pkg/kv/kvserver/kvserverbase/base.go line 143 at r1 (raw file):

Previously, wenyihu6 (Wenyi Hu) wrote…

Should we fall back to leases-and-replicas instead - LBRebalancingOff disables old store rebalancer as well. Added a commit for this.

I intentionally didn't so that we wouldn't fall someone who intentionally disabled lease rebalancing into lease rebalancing, but it doesn't matter, this works too.

The idea was that if you use the env var, the first thing you'd do after is to set the cluster setting to what you actually wanted, and then remove the env var.