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