Added check to postgres for instance stats and hot_standby replication lag
postgres checks for instance stats and hot_standby replication lag
addition of explicit var names and removal of uneeded loops
updated to reflect replication lag returned as timedelta
removed unneeded use of dictionaries to hold a single metric
removed unneeded use of dictionaries to hold a single metric
Merge pull request #952 from DataDog/postgresChecks

Added check to postgres for instance stats and hot_standby replication l...