Removed tracking of hostnames for metrics

This isn't hugely useful and mostly wastes InfluxDB space. We can re-add
this whenever needed (but only once we really need it).

config/initializers/metrics.rb

 if Gitlab::Metrics.enabled?
   require 'influxdb'
-  require 'socket'
   require 'connection_pool'
   require 'method_source'

lib/gitlab/metrics.rb

@@ -38,10 +38,6 @@ module Gitlab
       @pool
     end
-    def self.hostname
-      @hostname
-    end
-
     # Returns a relative path and line number based on the last application call
     # frame.
     def self.last_relative_application_frame
@@ -89,8 +85,6 @@ module Gitlab
       value.to_s.gsub('=', '\\=')
     end
-    @hostname = Socket.gethostname
-
     # When enabled this should be set before being used as the usual pattern
     # "@foo ||= bar" is _not_ thread-safe.
     if enabled?

lib/gitlab/metrics/metric.rb

@@ -17,10 +17,8 @@ module Gitlab
       # Returns a Hash in a format that can be directly written to InfluxDB.
       def to_hash
         {
-          series: @series,
-          tags:   @tags.merge(
-            hostname: Metrics.hostname
-          ),
+          series:    @series,
+          tags:      @tags,
           values:    @values,
           timestamp: @created_at.to_i * 1_000_000_000
         }

spec/lib/gitlab/metrics/metric_spec.rb

@@ -37,8 +37,6 @@ describe Gitlab::Metrics::Metric do
       it 'includes the tags' do
         expect(hash[:tags]).to be_an_instance_of(Hash)
-
-        expect(hash[:tags][:hostname]).to be_an_instance_of(String)
       end
       it 'includes the values' do

spec/lib/gitlab/metrics_spec.rb

@@ -13,12 +13,6 @@ describe Gitlab::Metrics do
     end
   end
-  describe '.hostname' do
-    it 'returns a String containing the hostname' do
-      expect(described_class.hostname).to eq(Socket.gethostname)
-    end
-  end
-
   describe '.last_relative_application_frame' do
     it 'returns an Array containing a file path and line number' do
       file, line = described_class.last_relative_application_frame