Bring prometheus source code into tezos/tezos but only as data

What

PART 1 - #7349 (closed)

Brings prometheus into the monorepo (the v1.2 release version).

This MR DOES NOT bring the entire history of prometheus into tezos/tezos. The reasoning behind this is that we do not need it, as we have not evolved our code according to changes from this project. So, we prefer to avoid polluting the commit history.

Why

Similar to data-encoding and resto, we want to have prometheus into the monorepo so developers can make MRs across the library boundary and avoid the release process. Another main reason is to facilitate the monorepoisation of cohttp, which will be proposed immediately afterwards, as prometheus-app is the only library that we use which depends on the cohttp-lwt/cohttp-lwt-unix libraries that we want to vendor.

By bringing it into tezos, we encourage tezos developers to pick up the maintenance and improvement of the project.

The MR only brings the code but doesn't do any of the integration so that it can be merged more easily. This is because the rebase process is complex.

How

prometheus/ is marked as data only in dune and is excluded from build file generation in manifest/main.ml

Manually testing the MR

This MR should have no impact apart from the directory prometheus/ being present. All current uses of the repository (make, other builds, tests, etc.) should work as usual. Consequently, in order to test you need to run all the commands you usually run.