pushing metrics with instance grouping_key would result: instance is reserved  #275

def push_metrics(registry)
  hostname = `hostname`.strip
  puts "pusing metrics"
  require 'prometheus/client/push'
  Prometheus::Client::Push.new(
    job: 'check_puppet',
    gateway: 'https://pushgateway.mydomain.com',
    grouping_key: { instance: hostname }
  ).add(registry)
end

/usr/local/lib64/ruby/gems/2.7.0/gems/prometheus-client-4.0.0/lib/prometheus/client/label_set_validator.rb:77:in `validate_reserved_key': instance is reserved (Prometheus::Client::LabelSetValidator::ReservedLabelError)
        from /usr/local/lib64/ruby/gems/2.7.0/gems/prometheus-client-4.0.0/lib/prometheus/client/label_set_validator.rb:32:in `block in validate_symbols!'
        from /usr/local/lib64/ruby/gems/2.7.0/gems/prometheus-client-4.0.0/lib/prometheus/client/label_set_validator.rb:29:in `each'
        from /usr/local/lib64/ruby/gems/2.7.0/gems/prometheus-client-4.0.0/lib/prometheus/client/label_set_validator.rb:29:in `all?'
        from /usr/local/lib64/ruby/gems/2.7.0/gems/prometheus-client-4.0.0/lib/prometheus/client/label_set_validator.rb:29:in `validate_symbols!'
        from /usr/local/lib64/ruby/gems/2.7.0/gems/prometheus-client-4.0.0/lib/prometheus/client/push.rb:35:in `initialize'