Add new Nginx 500 percentage metric

changelogs/unreleased/118788-add-nginx-500-percentage-metric.yml

+---
+title: Add Nginx error percentage metric
+merge_request: 28983
+author:
+type: added

config/prometheus/common_metrics.yml

@@ -218,3 +218,13 @@ panel_groups:
       query_range: 'sum(rate(nginx_server_requests{code="5xx", %{environment_filter}}[2m]))'
       label: HTTP Errors
       unit: "errors / sec"
+  - title: "HTTP Error Rate"
+    type: "area-chart"
+    y_label: "HTTP Errors (%)"
+    weight: 1
+    metrics:
+    - id: response_metrics_nginx_http_error_percentage
+      query_range: 'sum(rate(nginx_server_requests{code=~"5.*", host="*", %{environment_filter}}[2m])) / sum(rate(nginx_server_requests{code="total", host="*", %{environment_filter}}[2m])) * 100'
+      label: 5xx Errors (%)
+      unit: "%"
+

db/post_migrate/20200408110856_add_nginx_five_hundred_percentage_metric.rb

+# frozen_string_literal: true
+
+class AddNginxFiveHundredPercentageMetric < ActiveRecord::Migration[6.0]
+  DOWNTIME = false
+
+  def up
+    ::Gitlab::DatabaseImporters::CommonMetrics::Importer.new.execute
+  end
+
+  def down
+    # no-op
+  end
+end

db/structure.sql

@@ -13071,5 +13071,6 @@ COPY "schema_migrations" (version) FROM STDIN;
 20200403185422
 20200407094005
 20200407094923
+20200408110856
 \.
 

doc/user/project/integrations/prometheus_library/nginx.md

@@ -17,6 +17,7 @@ NGINX server metrics are detected, which tracks the pages and content directly s
 | Throughput (req/sec) | `sum(rate(nginx_server_requests{server_zone!="*", server_zone!="_", %{environment_filter}}[2m])) by (code)` |
 | Latency (ms) | `avg(nginx_server_requestMsec{%{environment_filter}})` |
 | HTTP Error Rate (HTTP Errors / sec) | `sum(rate(nginx_server_requests{code="5xx", %{environment_filter}}[2m]))` |
+| HTTP Error (%)| `sum(rate(nginx_server_requests{code=~"5.*", host="*", %{environment_filter}}[2m])) / sum(rate(nginx_server_requests{code="total", host="*", %{environment_filter}}[2m])) * 100` |
 
 ## Configuring Prometheus to monitor for NGINX metrics