slack 21.0: backport v22 VTOrc improvements, part 2

[timvaillancourt]

Description

This PR backports more v22 VTOrc fixes we run on slack-19.0:

1. vtorc: remove duplicate instance read from backend vitessio/vitess#17834
2. vtorc: add index for inst.ReadInstanceClusterAttributes table scan vitessio/vitess#17866
3. Add stats for shards watched by VTOrc, purge stale shards vitessio/vitess#17815
4. Remove unused code in discovery queue creation vitessio/vitess#17515
5. vtorc: cleanup discover queue, add concurrency flag vitessio/vitess#17825
6. vtorc: use errgroup in keyspace/shard discovery vitessio/vitess#17857
7. vtorc: skip unnecessary backend read in logic.LockShard(...) vitessio/vitess#17900
8. vtorc: add tablets watched stats vitessio/vitess#17911
9. vtorc: add stats for discovery workers vitessio/vitess#17937

Related Issue(s)

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

Deployment Notes

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @GuptaManan100 to sign the Salesforce Inc. Contributor License Agreement.