vtorc: fetch all tablets from cells once + filter during refresh

[timvaillancourt]

Description

This PR changes VTOrc tablet discovery to fetch all tablets once and filter them client side

Today, when --clusters_to_watch is set, VTOrc will call refreshTabletsInKeyspaceShard(...) in a goroutine for every shard when refreshing tablets at the interval --topo-information-refresh-duration

Unfortunately calling refreshTabletsInKeyspaceShard(...) is very inefficient because behind the scenes it is just loading all tablets from all cells because fetching just 1-shard or keyspace from the topo store isn't possible without fetching "all" 😱

This means that today a single refresh of all tablets causes shards x # tablets to be read instead of just # tablets, and much of the data read is just duplicated. Instead this PR causes only # of tablets to be fetched each tick

Related Issue(s)

Tracking issue: #17330

Checklist

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

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 86 lines in your changes missing coverage. Please review.

Project coverage is 67.51%. Comparing base (44aa9d7) to head (189b445).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtorc/logic/tablet_discovery.go	0.00%	82 Missing ⚠️
go/vt/vtorc/logic/vtorc.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17388      +/-   ##
==========================================
+ Coverage   67.47%   67.51%   +0.03%     
==========================================
  Files        1581     1581              
  Lines      253944   253972      +28     
==========================================
+ Hits       171353   171465     +112     
+ Misses      82591    82507      -84     

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[timvaillancourt]

This introduces a problem if shards are added/removed. Moving back to draft