Add support for a callback function when using an inserter

[prnvbn]

Use case

Getting additional telemetry for the inserter. Right now there is no way to know when a batch was committed. With the help of a callback function, one can hook arbitrary code to run after a batch has been committed

The main use case for this additional telemetry - e.g. bumping OTEL/prometheus metrics, adding log lines.

[prnvbn]

opened this PR #307

[prnvbn]

@abonander @slvrtrn please review the linked PR if you get the chance