Bump scheduled exports count only in case it has been scheduled

[arthurpassos]

Changelog category (leave one):

• Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Fixes the below by incrementing the scheduled counter only in case it has successfully scheduled the task

Medium: Scheduler slot counter increments after failed schedule attempt
Impact: Available background slots can be underutilized in the same scheduling cycle, delaying export throughput.

Partially fix #1498

Documentation entry for user-facing changes

...

CI/CD Options

Exclude tests:

• Fast test
• Integration Tests
• Stateless tests
• Stateful tests
• Performance tests
• All with ASAN
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• All with Aarch64
• All Regression
• Disable CI Cache

Regression jobs to run:

• Fast suites (mostly <1h)
• Aggregate Functions (2h)
• Alter (1.5h)
• Benchmark (30m)
• ClickHouse Keeper (1h)
• Iceberg (2h)
• LDAP (1h)
• Parquet (1.5h)
• RBAC (1.5h)
• SSL Server (1h)
• S3 (2h)
• Tiered Storage (2h)

[arthurpassos]

cc @Selfeer

[github-actions]

Workflow [PR], commit [f83f155]

[arthurpassos]

#1498