Adds pool_nodes.pg_status metric

[ngharo]

Adds a new guage for pg_status column in SHOW POOL_NODES; output.

# HELP pgpool2_pool_nodes_pg_status Backend pg status (1 for up or waiting, 0 for down or unused)
# TYPE pgpool2_pool_nodes_pg_status gauge
pgpool2_pool_nodes_pg_status{hostname="primary01.example.com",port="4432",role="primary"} 1
pgpool2_pool_nodes_pg_status{hostname="standby01.example.com",port="4432",role="standby"} 1
pgpool2_pool_nodes_pg_status{hostname="standby01.example.com",port="4432",role="standby"} 1

This is helpful for when you want to visualize frontend (pgpool) vs backend (postgres) status.