Validate alert's `payload` using JSON schema

Problem

The issue #222826 (closed) provides a list of JSONB columns which do NOT have a JSON schema to validate their content. See also https://docs.gitlab.com/ee/development/migration_style_guide.html#storing-json-in-database.

The content of AlertManagement::Alert#payload is currently not validated using a JsonSchemaValidator.

Alert's payload can have different shapes and is not static. We are currently supporting Custom payload for Generic alerts but also Prometheus alerts. With Custom Integrations (previously known as "Integration Builder") we will support even more highly customized formats and mappings definitions.

Proposed Solution

Validate the alert payload dynamically depending on its source (Generic alert, Prometheus, a custom mapping definition):

• Generic alerts - see static payload definition
• Prometheus alerts - see static payload definition
• Custom Integration - (hand-wavy) generate a JSON schema dynamically from the custom mapping definitions

Prerequisites

Currently, JsonSchemaValidator only supports static schema files located in app/validators/json_schemas/<file>.json. Example:

  class Alert
    validates :data, json_schema: { filename: "file" }
  end

Because of the different payloads' shapes we cannot use a single file. However, we could allow JsonSchemaValidator to retrieve the schema Hash from a method instead:

  class Alert
    validates :data, json_schema: { hash: :json_schema_hash }

    private

    def json_schema_hash
      case
      when generic? then generic_schema_hash
      when prometheus? then prometheus_schema_hash
      when custom_integration? then generate_custom_integration_schema_hash
      else
         ...
      end
    end
  end

Results/Outcome

After discussion between Peter and Kamil it was suggested NOT to support JSON validation for dynamic custom payloads (via "Custom Integrations").

Please see #227777 (comment 385950428) and #227777 (comment 386652194) more context.