Add configuration to for prometheus exporter (#5385)

**Story card:**
[sc-11693](https://app.shortcut.com/simpledotorg/story/11693/add-application-performance-monitoring)

## Because

We are migrating our monitoring infrastructure from datadog to
prometheus

## This addresses

This adds necessary instrumentation to monitor the various processes
using prometheus.

## Test instructions
- Start rails server
- In another terminal run `bundle exec prometheus-exporter`
- Visit the dashboard to send some requests to the server
- `curl http://localhost:9394/metrics`

Gemfile

@@ -94,6 +94,7 @@ gem "whenever", require: false
 gem "wkhtmltoimage-binary"
 gem "table_print"
 gem "doorkeeper", "~> 5.6"
+gem "prometheus_exporter"
 
 group :development, :test do
   gem "active_record_query_trace", require: false

Gemfile.lock

@@ -427,6 +427,8 @@ GEM
     prawn (2.4.0)
       pdf-core (~> 0.9.0)
       ttfunk (~> 1.7)
+    prometheus_exporter (2.1.0)
+      webrick
     prosopite (1.1.3)
     pry (0.14.1)
       coderay (~> 1.1)
@@ -687,6 +689,7 @@ GEM
       rack-proxy (>= 0.6.1)
       railties (>= 5.2)
       semantic_range (>= 2.3.0)
+    webrick (1.8.1)
     websocket (1.2.9)
     websocket-driver (0.7.5)
       websocket-extensions (>= 0.1.0)
@@ -779,6 +782,7 @@ DEPENDENCIES
   pg_query
   pg_search
   phonelib
+  prometheus_exporter
   prosopite
   pry-rails
   puma

config.ru

@@ -5,6 +5,8 @@ require_relative "config/environment"
 if defined?(PhusionPassenger)
   PhusionPassenger.on_event(:starting_worker_process) do |forked|
     if forked
+      require "prometheus_exporter/instrumentation"
+      PrometheusExporter::Instrumentation::Process.start(type: "web")
       Statsd.instance.reset!
     end
   end

config/initializers/prometheus.rb

@@ -0,0 +1,6 @@
+unless Rails.env.test?
+  require "prometheus_exporter/middleware"
+
+  # This reports stats per request like HTTP status and timings
+  Rails.application.middleware.unshift PrometheusExporter::Middleware
+end

config/initializers/sidekiq.rb

@@ -31,6 +31,14 @@ def self.connection_pool
 SIDEKIQ_STATS_PREFIX = "#{SimpleServer.env}.#{CountryConfig.current[:abbreviation]}"
 
 Sidekiq.configure_server do |config|
+  config.on :startup do
+    require "prometheus_exporter/instrumentation"
+    PrometheusExporter::Instrumentation::ActiveRecord.start(
+      custom_labels: {type: "sidekiq"},
+      config_labels: [:database, :host]
+    )
+  end
+
   config.on(:shutdown) { Statsd.instance.close }
 
   config.client_middleware do |chain|