[P1] event-exporter: add bounded queue + worker pool for sink batching

[erain]

Problem

The sink enqueues individual log entries into a single buffered channel and spawns a goroutine per flush. This makes throughput sensitive to burst patterns and offers limited visibility into queue depth or backpressure.

Proposed optimization

Introduce a bounded work queue + worker pool that batches by size/time with explicit queue depth metrics. This provides stable throughput under load and makes backpressure behavior observable.

Notes / references

• client-go workqueue (rate-limiting + metrics): https://pkg.go.dev/k8s.io/client-go/util/workqueue
• Prometheus metrics for queue depth/latency can mirror workqueue defaults.

Acceptance criteria

• Sink uses a bounded queue with worker pool and batcher.
• Metrics expose queue depth and batch flush latency.
• Throughput remains stable under bursty event streams.