Report hostname tag for Ruby VM probe metrics

The added hostname tag allows us to tell multiple hosts from each other
and graph them separately. If we do not, an app with multiple hosts
would have hosts overwrite one another's metrics. Only the last reported
value would be stored and shown.

I've copied some of the Sidekiq probe's and Puma plugin to determine the
hostname automatically and listen to the AppSignal config in case a
hostname was configured.

.changesets/track-hostname-tag-for-ruby-vm-metrics.md

@@ -0,0 +1,6 @@
+---
+bump: "patch"
+type: "fix"
+---
+
+Add hostname tag for Ruby VM metrics. This allows us to graph every host separately and multiple hosts won't overwrite each other metrics.

lib/appsignal/probes/mri.rb

@@ -62,7 +62,24 @@ def call
       private
 
       def set_gauge(metric, value, tags = {})
-        @appsignal.set_gauge(metric, value, tags)
+        @appsignal.set_gauge(metric, value, { :hostname => hostname }.merge(tags))
+      end
+
+      def hostname
+        return @hostname if defined?(@hostname)
+
+        config = @appsignal.config
+        @hostname =
+          if config[:hostname]
+            config[:hostname]
+          else
+            # Auto detect hostname as fallback. May be inaccurate.
+            Socket.gethostname
+          end
+        Appsignal.logger.debug "MRI probe: Using hostname config " \
+          "option '#{@hostname.inspect}' as hostname"
+
+        @hostname
       end
     end
   end

spec/lib/appsignal/probes/mri_spec.rb

@@ -1,17 +1,24 @@
 class AppsignalMock
   attr_reader :gauges
 
-  def initialize
+  def initialize(hostname: nil)
+    @hostname = hostname
     @gauges = []
   end
 
+  def config
+    ConfigHelpers.project_fixture_config.tap do |conf|
+      conf[:hostname] = @hostname if @hostname
+    end
+  end
+
   def set_gauge(*args) # rubocop:disable Naming/AccessorMethodName
     @gauges << args
   end
 end
 
 describe Appsignal::Probes::MriProbe do
-  let(:appsignal_mock) { AppsignalMock.new }
+  let(:appsignal_mock) { AppsignalMock.new(:hostname => hostname) }
   let(:probe) { described_class.new(appsignal_mock) }
 
   describe ".dependencies_present?" do
@@ -28,6 +35,8 @@ def set_gauge(*args) # rubocop:disable Naming/AccessorMethodName
 
   unless DependencyHelper.running_jruby? || DependencyHelper.running_ruby_2_0?
     describe "#call" do
+      let(:hostname) { nil }
+
       it "should track vm metrics" do
         probe.call
         expect_gauge_value("ruby_vm", :tags => { :metric => :class_serial })
@@ -73,17 +82,26 @@ def set_gauge(*args) # rubocop:disable Naming/AccessorMethodName
         expect_gauge_value("heap_slots", :tags => { :metric => :heap_live })
         expect_gauge_value("heap_slots", :tags => { :metric => :heap_free })
       end
+
+      context "with custom hostname" do
+        let(:hostname) { "my hostname" }
+
+        it "reports custom hostname tag value" do
+          probe.call
+          expect_gauge_value("heap_slots", :tags => { :metric => :heap_live, :hostname => hostname })
+        end
+      end
     end
   end
 
-  def expect_gauge_value(expected_key, expected_value = nil, tags: nil)
-    expected_tags = tags
+  def expect_gauge_value(expected_key, expected_value = nil, tags: {})
+    expected_tags = { :hostname => Socket.gethostname }.merge(tags)
     expect(appsignal_mock.gauges).to satisfy do |gauges|
       gauges.any? do |distribution_value|
         key, value, tags = distribution_value
         next unless key == expected_key
         next unless expected_value ? expected_value == value : !value.nil?
-        next if tags && tags != expected_tags
+        next unless tags == expected_tags
 
         true
       end

spec/support/helpers/config_helpers.rb

@@ -14,6 +14,7 @@ def project_fixture_config(env = "production", initial_config = {}, logger = App
       config_file
     )
   end
+  module_function :project_fixture_config, :project_fixture_path
 
   def start_agent(env = "production")
     Appsignal.config = project_fixture_config(env)