Path to PgBouncer pid file.
If provided, the standard process metrics get exported for the PgBouncer process, prefixed with 'pgbouncer_process_...'. The pgbouncer_process exporter needs to have read access to files owned by the PgBouncer process. Depends on the availability of /proc.
https://prometheus.io/docs/instrumenting/writing_clientlibs/#process-metrics.