Add timestamp option to postgres_extensible to handle log like queries.

[tfoldi]

Feature Request

Add timestamp option to postgres_extensible to handle log like queries.

I am about to send a PR, so you do not have to implement it - just don't hate that much

Proposal:

add timestamp option for query configuration such as:

  ## Structure :
  ## [[inputs.postgresql_extensible.query]]
  ##   sqlquery string
  ##   version string
  ##   withdbname boolean
  ##   tagvalue string (comma separated)
  ##   measurement string
  ##   timestamp string

in case timestamp column is defined, then the data point's timestamp will be parsed from that database column. If not specified, it will be as of today, using the current time.

Current behavior:

The data point's timestamp is always the current time.

Desired behavior:

Give users the possibility to decide if the timestamp can be retrieved from a column (just as you can do it for tag_set).

Use case:

Many apps log their actions into databases. In that case, each log line will be associated with the log insertion time instead of query execution time.

Simple use case, I want to quantify my Redshift Spectrum unload executions. The table definition is: https://docs.aws.amazon.com/redshift/latest/dg/r_STL_UNLOAD_LOG.html . If I want to check how many rows were inserted at what time, I need end_time as timestamp in my data point.

[sjwang90]

Closed in #8602