Add Table size metric

[diogonicoleti]

Added a new metric to return the total disk space in bytes used by the a table, including all indexes and TOAST data.

Sample output:

# HELP postgresql_table_size_bytes Total disk space in bytes used by the a table, including all indexes and TOAST data
# TYPE postgresql_table_size_bytes gauge
postgresql_table_size_bytes{database_name="postgres",table="mytable",type="index"} 81920
postgresql_table_size_bytes{database_name="postgres",table="mytable",type="table"} 73728
postgresql_table_size_bytes{database_name="postgres",table="mytable",type="toast"} 8192

refs https://github.com/ContaAzul/blackops/issues/1995
@ContaAzul/sre