feat: add auto-flush for periodic background metric emission

[MichelMajdalani]

• Add Builder::with_auto_flush() with 30s default interval
• Add Builder::with_auto_flush_interval() for custom intervals
• Spawn tokio background task with MissedTickBehavior::Skip
• Add tokio dependency (time, rt features)
• Refactor Builder::build() to return Collector directly
• Update documentation and examples

[bmorin]

Thank you for the contribution, nothing stood out as an issue, will look it over again tomorrow and likely merge it.

[peterallwin]

Should this also use spawn_graceful_shutdown_handler to ensure the final metrics are flushed on lambda termination?

[bmorin]

Should this also use spawn_graceful_shutdown_handler to ensure the final metrics are flushed on lambda termination?

Is flushing on the service being Poll::Ready(result) enough? That should flush once per invocation, right?

[peterallwin]

Should this also use spawn_graceful_shutdown_handler to ensure the final metrics are flushed on lambda termination?

Is flushing on the service being Poll::Ready(result) enough? That should flush once per invocation, right?

You're absolutely right, I initially interpreted the feature as replacing the per-invocation flushing with periodic flushing, should have reviewed it completely before commenting.