Remove broken metric for unique authors

This reverts commit eb6902ae.

We already have a similar metric:
https://metrics.gitlab.com/?q=redis_hll_counters.code_review.i_code_review_user_create_mr_monthly

Changelog: removed

config/metrics/counts_28d/20221108101211_merge_request_authors_monthly.yml

@@ -6,9 +6,11 @@ product_stage: create
 product_group: code_review
 product_category: code_review
 value_type: number
-status: active
+status: removed
 milestone: "15.6"
 introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/103334
+removed_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/106449
+milestone_removed: "15.7"
 time_frame: 28d
 data_source: database
 data_category: optional

lib/gitlab/usage/metrics/instrumentations/count_merge_request_authors_metric.rb

-# frozen_string_literal: true
-
-module Gitlab
-  module Usage
-    module Metrics
-      module Instrumentations
-        class CountMergeRequestAuthorsMetric < DatabaseMetric
-          operation :distinct_count, column: :author_id
-
-          relation { MergeRequest }
-        end
-      end
-    end
-  end
-end

spec/lib/gitlab/usage/metrics/instrumentations/count_merge_request_authors_metric_spec.rb

-# frozen_string_literal: true
-
-require 'spec_helper'
-
-RSpec.describe Gitlab::Usage::Metrics::Instrumentations::CountMergeRequestAuthorsMetric do
-  let(:expected_value) { 1 }
-  let(:start) { 30.days.ago.to_s(:db) }
-  let(:finish) { 2.days.ago.to_s(:db) }
-
-  let(:expected_query) do
-    "SELECT COUNT(DISTINCT \"merge_requests\".\"author_id\") FROM \"merge_requests\"" \
-    " WHERE \"merge_requests\".\"created_at\" BETWEEN '#{start}' AND '#{finish}'"
-  end
-
-  before do
-    user = create(:user)
-    user2 = create(:user)
-
-    create(:merge_request, created_at: 1.year.ago, author: user)
-    create(:merge_request, created_at: 1.week.ago, author: user2)
-    create(:merge_request, created_at: 1.week.ago, author: user2)
-  end
-
-  it_behaves_like 'a correct instrumented metric value and query', { time_frame: '28d' }
-end