Add alerts.libsonnet for generic alert patching, filtering and rendering

[simu]

The new alerts.libsonnet is a cleaned up copy of component-openshift4-monitoring's openshift4-monitoring-alert-patching.libsonnet + the core rendering logic of openshift4-monitoring-prom.libsonnet's generateRules() as renderRules().

The idea is that monitoring components can re-export this library with their global alert config parameter injected into field global_alert_params as follows:

(import 'syn/alerts.libsonnet') {
  global_alert_params+:: std.get(
    inv.parameters,
    'openshift4_monitoring',
    { alerts: {} }
  ).alerts,
} 

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[HappyTetrahedron]

LGTM. My brain's a bit too addled to look at the patching logic in detail so I primarily verified that the tests test the right things and the golden file looks as it should. afaiu the logic stayed the same as in component-openshift4-monitoring, right?

Wouldn't want to break the foo generator pipeline 🙃

[simu]

LGTM. My brain's a bit too addled to look at the patching logic in detail so I primarily verified that the tests test the right things and the golden file looks as it should. afaiu the logic stayed the same as in component-openshift4-monitoring, right?

Yeah, logic is copied from openshift4-monitoring and almost identical (I've added a call to filterRules() in renderGroups() which isn't there in the current prom.generateRules()).