[QueryThrottler]Replace polling with topo server watch for config updates

[siddharth16396]

Description

This PR Replaces periodic config polling (60s intervals) with event-driven topology server watches for QueryThrottler configuration updates. Instead of continuously reading config files on a timer, the throttler now subscribes to SrvKeyspace changes and reacts immediately when configs are updated in the topo server.

Changes Made

Removed the polling model:

• Deleted ConfigLoader interface and FileBasedConfigLoader implementation.
• Removed 60-second refresh loop from QueryThrottler.
• Eliminated file-based config loading entirely.

Added event-driven topo watch:

• Introduced startSrvKeyspaceWatch() that uses srvtopo.WatchSrvKeyspace() for real-time updates
• Added HandleConfigUpdate() callback for processing config changes
• Implemented smart deduplication to avoid redundant strategy reloads
• Added resilient error handling (continues watching on transient errors, stops only on permanent ones like NoNode)

New proto definitions:

• Created querythrottler.proto with Config, ThrottlingStrategy, and nested rule structures
• Added incoming_query_throttler_config field to SrvKeyspace message in topodata.proto
• Implemented ConfigFromProto() conversion between protobuf and internal config representation

Architecture improvements:

• Config now lives in topology server (centralized, versioned, observable)
• Tablets get instant updates when operators change throttling rules
• Initial config loaded synchronously during InitDBConfig() to ensure correct state on startup/restart
• Watch continues indefinitely with auto-retry for transient failures

How it works

1. Startup: InitDBConfig() sets the keyspace and immediately fetches initial config via GetSrvKeyspace()
2. Watch: Background goroutine subscribes to WatchSrvKeyspace() for the cell+keyspace
3. Updates: When config changes in topo, HandleConfigUpdate() fires, deduplicates, and hot-swaps the strategy
4. Resilience: Network blips are retried automatically; only fatal errors (context canceled, keyspace deleted) stop the watch

Benefits achieved:

• Faster propagation: Config changes apply immediately instead of waiting up to 60s
• Less load: No more periodic file reads from every tablet
• Better ops: Centralized config in topo server (can use vtctldclient to update)
• Consistency: All tablets see changes at roughly the same time
• Debugging: Topo changes are auditable (vs. scattered config files)

Related Issue(s)

• Question: v23.0.0 Error loading config: open /config/throttler-config.json: no such file or directory #18869

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• 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

[vitess-bot]

Review Checklist

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• Ensure that the Pull Request has a descriptive title.
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• Protobuf changes should be wire-compatible.
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[codecov]

Codecov Report

❌ Patch coverage is 77.58621% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.75%. Comparing base (79af4c1) to head (bb671ff).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
...let/tabletserver/querythrottler/query_throttler.go	72.00%	21 Missing ⚠️
...vttablet/tabletserver/querythrottler/test_utils.go	82.60%	4 Missing ⚠️
...t/tabletserver/querythrottler/registry/registry.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18905      +/-   ##
==========================================
+ Coverage   69.73%   69.75%   +0.01%     
==========================================
  Files        1608     1607       -1     
  Lines      214776   214839      +63     
==========================================
+ Hits       149781   149863      +82     
+ Misses      64995    64976      -19     

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[mattlord]

@siddharth16396 I like this! ❤️ I will review soon. In the meantime, after talking with @shlomi-noach ... (his idea), I think that for v23 we should just remove this log line altogether. The feature is still experimental, and you could leave the log message enabled in your fork if you like. But this way most users (virtually all), who are not using it, will not have this frequent (they might have aggregated logs across hundreds or even thousands of tablets) and likely confusing messages. What do you think? We can open that PR directly against the release-23.0 branch. If you prefer, I can also do that. Please let me know, as I'd like to do that soon so that we ensure it's in the first v23 patch release.

Thank you!

[siddharth16396]

I have this PR #18904
but i dont think its against v23 release branch, pls go ahead and remove the log.
And sorry for the troubles.