Count distinct authors of merge requests metric

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/379739

config/metrics/counts_28d/20221108101211_merge_request_authors_monthly.yml

+---
+key_path: usage_activity_by_stage_monthly.create.merge_request_authors_monthly
+description: Number of unique merge request authors
+product_section: dev
+product_stage: create
+product_group: code_review
+product_category: code_review
+value_type: number
+status: active
+milestone: "15.6"
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/103334
+time_frame: 28d
+data_source: database
+data_category: optional
+instrumentation_class: CountMergeRequestAuthorsMetric
+distribution:
+- ce
+- ee
+tier:
+- free
+- premium
+- ultimate

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