Add rubocop as development dependency

Re-introduce rubocop as per issue prometheus#132 and pin to a specific style guide
used by GoCardless and open-sourced at gocardless/gc_ruboconfig. This
should reduce the frequency that style guidelines change between
versions.

.rubocop.yml

@@ -0,0 +1,26 @@
+---
+inherit_gem:
+  gc_ruboconfig: rubocop.yml
+
+# Set this to be the oldest supported version of Ruby for this library. See
+# COMPATIBILITY.md for our compatibility and deprecation policy.
+AllCops:
+  TargetRubyVersion: 2.4
+
+Metrics/MethodLength:
+  Max: 15
+
+Style/RescueStandardError:
+  Exclude:
+    - "*/**/*_spec.rb"
+
+Naming/UncommunicativeMethodParamName:
+  AllowedNames:
+    # These are the default allowed names, set by Rubocop
+    - io
+    - id
+    # These are some custom names that we want to allow, since they aren't
+    # uncommunicative - they're actually rather meaningful!
+    - as
+    # This is our increment kwarg for Prometheus metrics
+    - by

prometheus-client.gemspec

@@ -1,4 +1,5 @@
-# -*- encoding: utf-8 -*-
+# frozen_string_literal: true
+
 $LOAD_PATH.push File.expand_path('../lib', __FILE__)
 require 'prometheus/client/version'
 
@@ -18,4 +19,5 @@ Gem::Specification.new do |s|
   s.add_dependency 'concurrent-ruby'
 
   s.add_development_dependency 'benchmark-ips'
+  s.add_development_dependency 'gc_ruboconfig', '= 2.4.0'
 end