Turn the get-hybrid monitoring config into a monitoring mixin

Monitoring mixins look like they could be becoming a standard way of sharing dashboards, rules and alerts. If we were to extract all of the jsonnet out of the runbooks to generate these files, we could publish our get-hybrid monitoring configuration.

With this, customers could use our dashboards and alerts to monitor their GitLab installation, and we might be able to use this more easily for Cells and dedicated.

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If I understand everything correctly, a mixin exposes 3 fields in jsonnet:

├── grafanaDashboards::
├── prometheusAlerts::
│   ├── groups:
├── prometheusRules::
│   ├── groups:
├── _config::

This is already very close with what our get-hybrid source looks like, the results are in generate.jsonnet

(import 'dashboards/dashboards.jsonnet')
+
(import 'rules/rules.jsonnet')

There each entry contains a yaml => content mapping, but we could keep this in jsonnet, and leave the rendering up to mixtool. We would have to split up rules into prometheusAlerts and prometheusRules. But I think this would be feasible

Our _config object is already extracted out into: gitlab-metrics-config.libsonnet, which we access all over the place. The way we work with this is by specifying the the gitlab-metrics-config we want to use on the right when we pass in the library paths for jsonnet. This allows dedicated (or the get-hybrid itself) to specify a different gitlab-metrics-config for a tenant.