Add created column to main cpanstats table

[preaction]

The main cpanstats table (CPAN::Testers::Schema::Result::Stats) has two existing date/time-ish fields:

• postdate, which is a YYYYMM stored as a mediumint
• fulldate, which is a YYYYMMDDHHNN stored as a varchar

Neither of these is ideal, since date/time comparisons in MySQL want ISO8601 date/time strings (which the datetime column can store). Using non-standard date/time formats makes it much harder to do ad-hoc reporting based on time periods (like pulling sections of the report summaries in Grafana dashboards, or for generating historical statistics for stats.cpantesters.org).

We should add a new column, created, to the cpanstats table. This column should be a datetime column, and should be initially populated based on the fulldate column (using 00 for the seconds field). This new column should be automatically populated by CPAN::Testers::Schema::ResultSet::Stats->insert_test_report, the standard API for inserting data into the cpanstats table.

It is possible to get a more accurate date/time for the report by using the test_reports table for new reports, or the metabase.metabase table for older reports, but the metabase table will make it much more difficult (and if knowing the seconds is that important, we can always fix it later).

Once we have this new column, we can start updating any other code that uses postdate or fulldate to instead use created (like CPAN::Testers::Schema::ResultSet::Release->since and CPAN::Testers::API::Controller::Summary->summary). While visiting other repositories, consider moving any useful code here to the schema.

[preaction]

For example, here's a SQL query I needed to write in Grafana:

SELECT
  CAST( UNIX_TIMESTAMP( STR_TO_DATE( fulldate, '%Y%m%d%H%i' ) ) / 900 AS signed ) * 900 as time,
  COUNT(tester) AS value,
  tester AS metric
FROM cpanstats
WHERE fulldate >= DATE_FORMAT( $__timeFrom(), '%Y%m%d%H%i' )
    AND fulldate <= DATE_FORMAT( $__timeTo(), '%Y%m%d%H%i' )
GROUP BY 1, tester
ORDER BY 1

I had to upgrade to Grafana 5.1, because the 4.3 we were on doesn't support $__timeFrom() and $__timeTo(), without which this would not be possible. If we had a simple created column, this query would be able to take full advantage of Grafana's SQL macros, and would look like:

SELECT
  $__timeGroup( created, '15m' ) as time,
  COUNT(tester) AS value,
  tester AS metric
FROM cpanstats
WHERE $__timeFilter( created )
GROUP BY 1, tester
ORDER BY 1