This adds:

* A `ScrapePoolConfig()` method to the scrape manager that allows getting
  the scrape config for a given pool.
* An API endpoint at `/api/v1/targets/relabel_steps` that takes a pool name
  and a label set of a target and returns a detailed list of applied
  relabeling rules and their output for each step.
* A "show relabeling" link/button for each target on the discovery page
  that shows the detailed flow of all relabeling rules (based on the API
  response) for that target.

Note that this changes the JSON encoding of the relabeling rule config
struct to output the original snake_case (instead of camelCase) field names,
and before merging, we need to be sure that's ok :) See my comment about
that at #15383 (comment)

Fixes #17283

Signed-off-by: Julius Volz <julius.volz@gmail.com>

juliusv deleted the ui/visualize-relabel-steps branch

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