Fix pid providing for Prometheus

Use relative worker identifier for metrics (instead of Process.pid) and
identify when Unicorn/Puma/Sidekiq is used.
Previously, it was assumed that all metrics are gathered from Unicorn
due to hardcoded implementation which was incorrect.

config/initializers/7_prometheus_metrics.rb

@@ -1,5 +1,4 @@
 require 'prometheus/client'
-require 'prometheus/client/support/unicorn'
 
 # Keep separate directories for separate processes
 def prometheus_default_multiproc_dir
@@ -23,7 +22,7 @@ Prometheus::Client.configure do |config|
 
   config.multiprocess_files_dir = ENV['prometheus_multiproc_dir'] || prometheus_default_multiproc_dir
 
-  config.pid_provider = Prometheus::Client::Support::Unicorn.method(:worker_pid_provider)
+  config.pid_provider = Prometheus::PidProvider.method(:worker_id)
 end
 
 Gitlab::Application.configure do |config|

lib/prometheus/pid_provider.rb

@@ -0,0 +1,42 @@
+# frozen_string_literal: true
+
+require 'prometheus/client/support/unicorn'
+
+module Prometheus
+  module PidProvider
+    extend self
+
+    def worker_id
+      if Sidekiq.server?
+        'sidekiq'
+      elsif defined?(Unicorn::Worker)
+        "unicorn_#{unicorn_worker_id}"
+      elsif defined?(::Puma)
+        "puma_#{puma_worker_id}"
+      else
+        "process_#{Process.pid}"
+      end
+    end
+
+    private
+
+    # This is not fully accurate as we don't really know if the nil returned
+    # is actually means we're on master or not.
+    # Follow up issue was created to address this problem and
+    # to introduce more structrured approach to a current process discovery:
+    # https://gitlab.com/gitlab-org/gitlab-ce/issues/64740
+    def unicorn_worker_id
+      ::Prometheus::Client::Support::Unicorn.worker_id || 'master'
+    end
+
+    # See the comment for #unicorn_worker_id
+    def puma_worker_id
+      match = process_name.match(/cluster worker ([0-9]+):/)
+      match ? match[1] : 'master'
+    end
+
+    def process_name
+      $0
+    end
+  end
+end

spec/lib/prometheus/pid_provider_spec.rb

@@ -0,0 +1,79 @@
+# frozen_string_literal: true
+
+require 'fast_spec_helper'
+
+describe Prometheus::PidProvider do
+  describe '.worker_id' do
+    subject { described_class.worker_id }
+
+    let(:sidekiq_module) { Module.new }
+
+    before do
+      allow(sidekiq_module).to receive(:server?).and_return(false)
+      stub_const('Sidekiq', sidekiq_module)
+    end
+
+    context 'when running in Sidekiq server mode' do
+      before do
+        expect(Sidekiq).to receive(:server?).and_return(true)
+      end
+
+      it { is_expected.to eq 'sidekiq' }
+    end
+
+    context 'when running in Unicorn mode' do
+      before do
+        stub_const('Unicorn::Worker', Class.new)
+        hide_const('Puma')
+      end
+
+      context 'when `Prometheus::Client::Support::Unicorn` provides worker_id' do
+        before do
+          expect(::Prometheus::Client::Support::Unicorn).to receive(:worker_id).and_return(1)
+        end
+
+        it { is_expected.to eq 'unicorn_1' }
+      end
+
+      context 'when no worker_id is provided from `Prometheus::Client::Support::Unicorn`' do
+        before do
+          expect(::Prometheus::Client::Support::Unicorn).to receive(:worker_id).and_return(nil)
+        end
+
+        it { is_expected.to eq 'unicorn_master' }
+      end
+    end
+
+    context 'when running in Puma mode' do
+      before do
+        stub_const('Puma', Module.new)
+        hide_const('Unicorn::Worker')
+      end
+
+      context 'when cluster worker id is specified in process name' do
+        before do
+          expect(described_class).to receive(:process_name).and_return('puma: cluster worker 1: 17483 [gitlab-puma-worker]')
+        end
+
+        it { is_expected.to eq 'puma_1' }
+      end
+
+      context 'when no worker id is specified in process name' do
+        before do
+          expect(described_class).to receive(:process_name).and_return('bin/puma')
+        end
+
+        it { is_expected.to eq 'puma_master' }
+      end
+    end
+
+    context 'when running in unknown mode' do
+      before do
+        hide_const('Puma')
+        hide_const('Unicorn::Worker')
+      end
+
+      it { is_expected.to eq "process_#{Process.pid}" }
+    end
+  end
+end