[FEATURE] hidden receivers in UI/API

[cxdy]

Description

This pull request does the following:

• Adds the ability to "hide" alerts from specific receivers in the UI and API responses
• Adds a button to the UI to show alerts from "hidden" receivers
• Adds a URL parameter ?includeHidden=true to show "hidden" receivers in both UI and API responses

This is set in the receivers: block, like this:

receivers:
- name: 'webhook-receiver'
  hidden: true
  webhook_configs:
  - url: 'http://webhook-receiver:8080/alerts'

If unset, it will not be hidden.

Why?

This may be a niche usecase, but in our environment, we have a webhook receiver (internal tool) that is our top-level route, thus it receives every alert that reaches our Alertmanager cluster. This means that all alerts are essentially duplicated in the UI response when hitting the /alerts endpoint.

root@prometheus1:~# curl -s http://127.0.0.1:9093//api/v2/alerts | jq '[.[] | {receiver: .receivers[].name}] | group_by(.receiver) | map({receiver: .[0].receiver, count: length}) | sort_by(-.count) | .[0:1]'
[
  {
    "receiver": "webhook-receiver",
    "count": 8344
  }
]

This makes the UI load a bit slower than we'd like, and other folks use the Alertmanager API for various automations and have been complaining of poor performance due to this (amongst other things).

Checklist

• Is this a new feature?
  • I have added tests that test the new feature's functionality
• I have added/updated the required documentation
• I have signed-off my commits
• I will follow best practices for contributing to this project