Add OpenTelemetry instrumentation support via enable-otel flag

[tonyalaribe]

When the flag is enabled, database operations automatically create OpenTelemetry spans using hs-opentelemetry-instrumentation-postgresql-simple. No code changes required. The same Effectful.PostgreSQL module transparently uses instrumented functions.

Includes workaround for forEach bug in the upstream instrumentation package.

[fpringle]

Thanks for the contribution! Just a few questions.

I'll add some commits afterwards - the CICD should test the build both with and without opentelemetry, and the Nix overlay should include the revision you pinned in cabal.project.

[fpringle]

What bug is this? If there's a Github issue on the hs-opentelemetry repo, I can follow it and make sure to update this line when it's patched.

[fpringle]

Could you elaborate on why this is needed? Will the Hackage version eventually catch up, so we'll be able to remove this?

I should probably add this to the nix overlay as well.

[fpringle]

Another thought: I'm not sure about controlling the OpenTelemetry integration via a cabal flag (which are a bit fragile, and slightly difficult to control when not building directly with cabal). Before this, there was only one reasonable way to do each of the PostgreSQL operations inside the WithConnection effect: by just delegating to the corresponding postgresql-simple library. But now, there are 2. In effectful, when I hear "multiple ways to perform an effect", I think "dynamic effect". This would also give us the option to decide whether or not to use the OpenTelemetry integration at runtime.

I'm going to have a go at wrapping all the PostgreSQL operations (forEach, insert etc) into a dynamic effect, which can then be interepreted by multiple interpreters using WithConnection. Then I'll see how that feels.