WIP: Add finder for prometheus metrics

What does this MR do?

Adds a finder for the standard PrometheusMetrics model. This was a follow-up item from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31063#note_200076126.

Updates references to prometheus metrics to use the finder. Note that the PrometheusMetric model used for importing common metrics into the db is different and not related to this work.

Issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/65753

EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/16235

Does this MR meet the acceptance criteria?

Conformity

• [n/a] Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [n/a] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

app/finders/prometheus_metrics_finder.rb

 # frozen_string_literal: true
 
 class PrometheusMetricsFinder
-  attr_reader :params
+  ACCEPTED_PARAMS = [
+    :project,
+    :group,
+    :title,
+    :y_label,
+    :identifier,
+    :common,
+    :order_by
+  ].freeze
 
-  # @params [Hash<Symbol, any>] representing any prometheus metrics column
   def initialize(params = {})
+    validate_params!(params)
+
     @params = params
   end
 
-  # rubocop: disable CodeReuse/ActiveRecord
   def execute
-    PrometheusMetric.where(params)
+    # Scopes
+    metrics = by_project(::PrometheusMetric)
+    metrics = by_group(metrics)
+    metrics = by_title(metrics)
+    metrics = by_y_label(metrics)
+    metrics = common(metrics)
+    metrics = ordered(metrics)
+
+    # Finds
+    metrics = find_identifier(metrics)
+
+    metrics
+  end
+
+  private
+
+  attr_reader :params
+
+  def by_project(metrics)
+    return metrics unless params[:project]
+
+    metrics.for_project(params[:project])
+  end
+
+  def by_group(metrics)
+    return metrics unless params[:group]
+
+    metrics.for_group(params[:group])
+  end
+
+  def by_title(metrics)
+    return metrics unless params[:title]
+
+    metrics.for_title(params[:title])
+  end
+
+  def by_y_label(metrics)
+    return metrics unless params[:y_label]
+
+    metrics.for_y_label(params[:y_label])
+  end
+
+  def common(metrics)
+    return metrics unless params[:common]
+
+    metrics.common
+  end
+
+  def ordered(metrics)
+    return metrics unless params[:order_by]
+
+    metrics.order(params[:order_by])
+  end
+
+  def find_identifier(metrics)
+    return metrics unless params[:identifier]
+
+    metrics.for_identifier(params[:identifier]).first
+  end
+
+  def validate_params!(params)
+    raise ArgumentError, "Please provide one or more of: #{ACCEPTED_PARAMS}" if params.blank?
+    raise ArgumentError, ":identifier must be scoped to a :project or :common" if params[:identifier] && !(params[:project] || params[:common])
   end
-  # rubocop: enable CodeReuse/ActiveRecord
 end