Add stats for shards watched by VTOrc, purge stale shards

[timvaillancourt]

Description

This PR adds stats for what keyspace/shards are being watched by VTOrc, populated using the backend database

While adding this support I noticed records are never deleted from the backend vitess_shard table, so I added support for stale records to be deleted in a similar way to tablets: by recording what was successfully "saved" in the latest polling and removing anything that was not in that list

Related Issue(s)

Closes #17816

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• 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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[timvaillancourt]

refreshAllShards does a save + read + delete, per keyspace. Lets make sure that doesn't happen concurrently

[codecov]

Codecov Report

Attention: Patch coverage is 46.66667% with 32 lines in your changes missing coverage. Please review.

Project coverage is 67.43%. Comparing base (70114ad) to head (474115d).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtorc/logic/keyspace_shard_discovery.go	0.00%	32 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17815      +/-   ##
==========================================
- Coverage   67.94%   67.43%   -0.52%     
==========================================
  Files        1586     1592       +6     
  Lines      255224   258136    +2912     
==========================================
+ Hits       173420   174068     +648     
- Misses      81804    84068    +2264     

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[timvaillancourt]

This is working nicely on our fork:

vitess@REDACTED:/vt$ curl -s localhost:15000/metrics | grep vtorc_shards_watched | head -10
# HELP vtorc_shards_watched Keyspace/shards currently watched
# TYPE vtorc_shards_watched gauge
vtorc_shards_watched{keyspace="REDACTED",shard="-0040"} 1
vtorc_shards_watched{keyspace="REDACTED",shard="0040-0080"} 1
vtorc_shards_watched{keyspace="REDACTED",shard="0080-00c0"} 1
vtorc_shards_watched{keyspace="REDACTED",shard="00c0-0100"} 1
vtorc_shards_watched{keyspace="REDACTED",shard="0100-0140"} 1
vtorc_shards_watched{keyspace="REDACTED",shard="0140-0180"} 1
vtorc_shards_watched{keyspace="REDACTED",shard="0180-01c0"} 1
vtorc_shards_watched{keyspace="REDACTED",shard="01c0-0200"} 1