0.2

First really usable release!

• require pbrt only, use ocaml-protoc as a lint
• add Metrics_callbacks module
• add histogram metrics (untested)
• basic support for logs
• expose tick function (useful in the absence of a background thread)
• debug the library is set via OTEL_OCAML_DEBUG
• provide conventions for standard metrics
• add runtime attrs to GC stats