Add hs-opentelemetry

[isovector]

This PR adds an --enable-opentelemetry flag which, when enabled, turns on telemetry around hackage-server's router.

The purpose here is to start collecting traces and get real data on which routes get used in production, and how expensive each is.

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[phadej]

while telemetry could be useful, I think that hs-opentelemetry-sdk is a liability (iand675/hs-opentelemetry#315 - no bounds), especially for a project with effectively zero active maintainers.

[ysangkok]

I think logging/monitoring would be great to have, even if the project is under-maintained. But since I do not work on deployments, I will defer to @gbaz on whether this is the right direction.

[isovector]

@phadej: very reasonable point. Would hiding it behind a default-off flag assuage some fears here? The reason I ask is that I'm in the middle of chicken-and-egg situation here; I'm trying to get enough data in order to determine if hackage-server-v3 can meaningfully move the needle. Which if it does, is a good argument to continue moving forwards on it.

Maybe we could gather this information from logs on the existing load balancer / reverse proxy? Perhaps @gbaz would know?