prometheus: Optionally remove service discovery.

I read this hilarious blog post:

https://wejick.wordpress.com/2022/01/29/can-i-have-a-smaller-prometheus/

We can have a smaller Prometheus too. This patch allows users to remove
service discovery for five public clouds (AWS, Azure, DigitalOcean, GCP,
and Linode) and also Kubernetes, simply by setting the corresponding
enable-flag to `false`. I have tested building with each flag as I added
it to the list. I also tested running with all six flags set to `false`,
and the resulting Prometheus can still handle my orthogonal
service-discovery configuration (files).

To meet Adam Savage's definition of science, I measured the size of the
`prometheus` and `promtool` binaries after adding each flag with
`ls -h`.

flag          | prometheus | promtool
--------------|------------|----------
starting size | 84M        | 74M
AWS           | 72M        | 61M
Azure         | 71M        | 61M
GCE           | 64M        | 53M
k8s           | 40M        | 53M
DO            | 39M        | 52M
Linode        | 38M        | 51M

I did not go as far as the blog post. If folks want, I'll make the rest
of the service discovery optional too.

I did not shrink the build closure, just the output closure; we still
pull all of the various vendored modules into the Nix store during
builds. I don't see how to do this in a neat or easy way.

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