Expose gc-stats as an API

[Sudha247]

Olly currently exposes a CLI interface for gc-stats. It's possibly useful to expose this APIs that other programs can use. For instance, meio re-implements some functionality (https://github.com/ocaml-multicore/meio/blob/main/src/lib/latency.ml). Making APIs available would let meio use it directly from olly.

cc @patricoferris