Only start flush logs timer when there's an event to write

[cjaliaga]

The timer used to periodically flush logs for DiagnosticEvents is currently initialized as soon as the singleton Repository is created, regardless of whether any events are ever written. With the change in Service Provider implementation in the OOP host, this behavior changed: singletons are now initialized eagerly at startup by JobHostScopedServiceProviderFactory.

azure-functions-host/src/WebJobs.Script.WebHost/DependencyInjection/JobHostScopedServiceProviderFactory.cs

Line 68 in 575890a

var jobHostServices = _rootProvider.CreateChildContainer(_rootServices);

resolves #11098

Changes:

• Transitions the _flushLogsTimer to a lazy initialization pattern, ensuring the timer is only started when there's an event to write.
• Changed logging levels from Error to Warning to better reflect the severity of the issues.
• Disabled the service when there's any failure (even if transient) as part of flush logs, purge events or execute batch.

Pull request checklist

IMPORTANT: Currently, changes must be backported to the in-proc branch to be included in Core Tools and non-Flex deployments.

• Backporting to the in-proc branch is not required
  • Otherwise: Link to backporting PR
• My changes do not require documentation changes
  • Otherwise: Documentation issue linked to PR
• My changes should not be added to the release notes for the next release
  • Otherwise: I've added my notes to release_notes.md
• My changes do not need to be backported to a previous version
  • Otherwise: Backport tracked by issue/PR #issue_or_pr
• My changes do not require diagnostic events changes
  • Otherwise: I have added/updated all related diagnostic events and their documentation (Documentation issue linked to PR)
• I have added all required tests (Unit tests, E2E tests)

[jviau]

Approving. Changing the log level could be considered a breaking change (a very minor one at that). But I think the recent customer issues warrants the change.