flow based tablet load balancer (#16351)

[venkatraju]

Description

Backport of flow based tablet balancer from upstream PR

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

[timvaillancourt]

@venkatraju this unit test failure looks new:

2024-08-29T22:37:24.0601578Z --- FAIL: TestGetKeyspaceShardsToWatch (0.00s)
2024-08-29T22:37:24.0601911Z     --- FAIL: TestGetKeyspaceShardsToWatch/multi_keyspace (0.00s)
2024-08-29T22:37:24.0602115Z         tablet_discovery_test.go:400: 
2024-08-29T22:37:24.0602775Z             	Error Trace:	/home/runner/work/vitess/vitess/go/vt/vtorc/logic/tablet_discovery_test.go:400
2024-08-29T22:37:24.0602974Z             	Error:      	Not equal: 
2024-08-29T22:37:24.0605690Z             	            	expected: []*topo.KeyspaceShard{(*topo.KeyspaceShard)(0xc000894ac0), (*topo.KeyspaceShard)(0xc000894ae0), (*topo.KeyspaceShard)(0xc000894b00), (*topo.KeyspaceShard)(0xc000894b20), (*topo.KeyspaceShard)(0xc000894b40), (*topo.KeyspaceShard)(0xc000894b60), (*topo.KeyspaceShard)(0xc000894b80), (*topo.KeyspaceShard)(0xc000894ba0)}
2024-08-29T22:37:24.0608371Z             	            	actual  : []*topo.KeyspaceShard{(*topo.KeyspaceShard)(0xc0008953a0), (*topo.KeyspaceShard)(0xc0008953c0), (*topo.KeyspaceShard)(0xc0008953e0), (*topo.KeyspaceShard)(0xc000895420), (*topo.KeyspaceShard)(0xc000895440), (*topo.KeyspaceShard)(0xc000895460), (*topo.KeyspaceShard)(0xc000895560), (*topo.KeyspaceShard)(0xc0008955a0)}
2024-08-29T22:37:24.0608553Z             	            	
2024-08-29T22:37:24.0608749Z             	            	Diff:
2024-08-29T22:37:24.0609109Z             	            	--- Expected
2024-08-29T22:37:24.0609332Z             	            	+++ Actual
2024-08-29T22:37:24.0609699Z             	            	@@ -1,10 +1,2 @@
2024-08-29T22:37:24.0610096Z             	            	 ([]*topo.KeyspaceShard) (len=8) {
2024-08-29T22:37:24.0610466Z             	            	- (*topo.KeyspaceShard)({
2024-08-29T22:37:24.0611167Z             	            	-  Keyspace: (string) (len=14) "test_keyspace2",
2024-08-29T22:37:24.0611634Z             	            	-  Shard: (string) (len=9) "1100-1200"
2024-08-29T22:37:24.0611870Z             	            	- }),
2024-08-29T22:37:24.0612238Z             	            	- (*topo.KeyspaceShard)({
2024-08-29T22:37:24.0612756Z             	            	-  Keyspace: (string) (len=14) "test_keyspace2",
2024-08-29T22:37:24.0613195Z             	            	-  Shard: (string) (len=9) "1200-1300"
2024-08-29T22:37:24.0613426Z             	            	- }),
2024-08-29T22:37:24.0613752Z             	            	  (*topo.KeyspaceShard)({
2024-08-29T22:37:24.0614080Z             	            	@@ -32,2 +24,10 @@
2024-08-29T22:37:24.0614484Z             	            	   Shard: (string) (len=3) "80-"
2024-08-29T22:37:24.0614681Z             	            	+ }),
2024-08-29T22:37:24.0615003Z             	            	+ (*topo.KeyspaceShard)({
2024-08-29T22:37:24.0615462Z             	            	+  Keyspace: (string) (len=14) "test_keyspace2",
2024-08-29T22:37:24.0615912Z             	            	+  Shard: (string) (len=9) "1100-1200"
2024-08-29T22:37:24.0616105Z             	            	+ }),
2024-08-29T22:37:24.0616430Z             	            	+ (*topo.KeyspaceShard)({
2024-08-29T22:37:24.0616959Z             	            	+  Keyspace: (string) (len=14) "test_keyspace2",
2024-08-29T22:37:24.0617409Z             	            	+  Shard: (string) (len=9) "1200-1300"
2024-08-29T22:37:24.0617600Z             	            	  })
2024-08-29T22:37:24.0617967Z             	Test:       	TestGetKeyspaceShardsToWatch/multi_keyspace
2024-08-29T22:37:24.0618053Z FAIL
2024-08-29T22:37:24.0618221Z FAIL	vitess.io/vitess/go/vt/vtorc/logic	1.044s

The 4 x downgrade query serving CI can be ignored, however

[timvaillancourt]

Some CI failures need addressing