Removed various default metrics tags

While it's useful to keep track of the different versions (Ruby, GitLab,
etc) doing so for every point wastes disk space and possibly also RAM
(which InfluxDB is all to eager to gobble up). If we want to see the
performance differences between different GitLab versions simply looking
at the performance since the last release date should suffice.

lib/gitlab/metrics/metric.rb

@@ -20,9 +20,6 @@ module Gitlab
           series: @series,
           tags:   @tags.merge(
             hostname:     Metrics.hostname,
-            ruby_engine:    RUBY_ENGINE,
-            ruby_version:   RUBY_VERSION,
-            gitlab_version: Gitlab::VERSION,
             process_type: Sidekiq.server? ? 'sidekiq' : 'rails'
           ),
           values:    @values,

spec/lib/gitlab/metrics/metric_spec.rb

@@ -39,9 +39,6 @@ describe Gitlab::Metrics::Metric do
         expect(hash[:tags]).to be_an_instance_of(Hash)
 
         expect(hash[:tags][:hostname]).to be_an_instance_of(String)
-        expect(hash[:tags][:ruby_engine]).to be_an_instance_of(String)
-        expect(hash[:tags][:ruby_version]).to be_an_instance_of(String)
-        expect(hash[:tags][:gitlab_version]).to be_an_instance_of(String)
         expect(hash[:tags][:process_type]).to be_an_instance_of(String)
       end