Improve Query Time Resolution in QAN

[gordan-bobic]

Currently, the queries get logged as having executed at the time when they were harvested from the slow log, not when they executed. In most, probably all cases, slow log contains a SET TIMESTAMP statement with the timestamp of when the query was executed.

CREATE TABLE `ssm`.`query_class_metrics` (
  `query_class_id` int(10) unsigned NOT NULL,
  `instance_id` int(10) unsigned NOT NULL,
  `start_ts` timestamp NOT NULL DEFAULT current_timestamp(),
  ...
  PRIMARY KEY (`query_class_id`,`instance_id`,`start_ts`),
  KEY `start_ts` (`start_ts`),
  KEY `instance_start` (`instance_id`,`start_ts`)
);

CREATE TABLE `ssm`.`query_global_metrics` (
  `instance_id` int(10) unsigned NOT NULL,
  `start_ts` timestamp NOT NULL DEFAULT current_timestamp(),
  ...
  PRIMARY KEY (`instance_id`,`start_ts`),
  KEY `start_ts` (`start_ts`)
);

That means our time resolution for queries is the length of time it takes QAN to process a batch of queries. We could improve this to 1 second by making use of the timestamp information.

To maintain behaviour similar to what happens currently, the query should be logged with start_ts of (SET TIMESTAMP + Query_time) listed in the slow log, rounded to nearest second (to evade the need to change start_ts column type. Since the timestamp in the slow log only has resolution of 1 second, switching to timestamp(6) would only improve real resolution by approximately half a second (because we would only get the fraction part of the Query_time), and that small improvement doesn't seem to justify the cost of converting potentially tens or hundreds of gigabytes of QAN tables.

Column name start_ts is misleading since this logs when the query was loaded, which is after it finished rather than when it started. But we can ignore this for now.