Add missing `pid=` for metric without labels

This fixes the duplicate metrics returned, if the metric is not the livesum type of metric.

The metrics should always be annotated with pid in such cases, but there's a bug in code that makes it append pid only when metric has labels. In multi-proc environment, not all metrics do have labels.

This fixes:

ruby_process_start_time_seconds 19233
ruby_process_start_time_seconds 19233

To be:

ruby_process_start_time_seconds{pid="unicorn_0"} 19233
ruby_process_start_time_seconds{pid="unicorn_1"} 19233