Side Inputs + Blob Store Backups - Part 1 - Fix duplicate uploads and broken metrics

[prateekm]

Part 1 of 2. Follow up PR to restore side input stores using Blob Store backups coming soon.

Symptoms:

1. Side input stores are uploaded twice when using Blob Store State Backend.
2. Store-level Gauges (but not Timers) in BlobStoreBackupManagerMetrics are broken for side input stores.
3. Task level Gauges in BlobStoreBackupManagerMetrics have incorrect value (count twice for side input stores).

Cause:

1. StorageConfig#getStoreNames() returns side input stores twice in the list.
2. BlobStoreBackupManager does not dedup storesToBackup list.
3. PR SAMZA-2397: Updating gauge-val function on newGauge on same metric name #1223 makes the duplicate-registration behavior between Gauges and Timers inconsistent.

Changes:

1. Fixed StorageConfig#getStoreNames() to dedup store names.
2. Added defensive dedup in BlobStoreBackupManager.
3. Changed store level metrics initialization in BlobStoreBackupManagerMetrics to computeIfAbsent instead of putIfAbsent to avoid overwriting-yet-returning-old-Gauges in case of duplicate store names.

Tests:
Added unit tests for StorageConfig to verify deduping.

[prateekm]

cc @shekhars-li for review.

[shekhars-li]

Thanks a lot for fixing it.