Use SdkClient for persistence in MonitorMetadataService

[manaswini1920]

Description

Migrates MonitorMetadataService.upsertMetadata() and getMetadata() to use SdkClient instead of direct OpenSearch client calls, as part of the remote metadata SDK migration for background services.

Changes:

• MonitorMetadataService.upsertMetadata(): Replaced IndexRequest with PutDataObjectRequest via sdkClient.putDataObjectAsync()
• MonitorMetadataService.getMetadata(): Replaced GetRequest with GetDataObjectRequest via sdkClient.getDataObjectAsync()
• AlertingPlugin.kt: Set sdkClient on MonitorMetadataService after SDK client creation
• SdkUtils.kt: Added suspend fun CompletionStage.await() helper for coroutine-safe SDK calls (same as Use SdkClient for persistence in IndexAlertingComment, IndexMonitor #2081)

Notes:

• createFullRunContext() calls (getIndex, indicesStats) are admin/infrastructure operations and remain on direct client
• Routing was previously used on SCHEDULED_JOBS_INDEX but is optional (not _routing.required: true), so SDK migration works without routing support
• Depends on Use SdkClient for persistence in IndexAlertingComment, IndexMonitor #2081 for SdkUtils.kt

Related Issues

Part of remote metadata SDK migration for background services.

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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