Expose metrics

[pavolloffay]

For fine-tuning parameters like

# Batch size. Default 10_000.
batch_write_size:
# Batch flush size. Default 5s.
batch_flush_interval:

It would be great to expose metrics like:

• batch flush size
• number of flushes due to timer expiration (batch_flush_interval)

Although, I am not sure if the batch size influences performance or not.