Add new metrics for long running transactions

[joaofoltran]

Updated both metrics for long running transactions. One returns 4 metrics (one for each threshold, 1min, 5min, 10min, 30min) and another one returns the duration of the longest running transaction.

With this we can check if there was a long running transaction during the time of any issues that could cause xmin/lsn retention.

The original code was just counting how many transactions and then bringing the longest running one. This new code we know if there are multiple long running ones.

Next I'll be adding these to our grafana metrics so we can check them when diagnosing issues.

[mble]

med: I don't love this. We can re-express all this in a single query:

SELECT
  count(*) FILTER (
    WHERE
      extract('epoch', clock_timestamp() - xact_start) >= 60
  )
    AS count_60,
  count(*) FILTER (
    WHERE
      extract('epoch', clock_timestamp() - xact_start)
      >= 300
  )
    AS count_300,
  count(*) FILTER (
    WHERE
      extract('epoch', clock_timestamp() - xact_start)
      >= 600
  )
    AS count_600,
  count(*) FILTER (
    WHERE
      extract('epoch', clock_timestamp() - xact_start)
      >= 1800
  )
    AS count_1800,
  COALESCE(
    max(extract('epoch', clock_timestamp() - xact_start)),
    0
  )
    AS oldest_timestamp_seconds
FROM
  pg_catalog.pg_stat_activity
WHERE
  state IS DISTINCT FROM 'idle'
  AND query NOT LIKE 'autovacuum:%'
  AND xact_start IS NOT NULL;

Can then break it up based out of the results.

[joaofoltran]

oh, yes, much better, will update :)

[mble]

med: With the updated query, we can use QueryRowContext instead to avoid manual row closure handling.

[joaofoltran]

Updated applying your changes @mble , check it out see if it improved :)
(has been quite some time since I've touched golang)

[mble]

@joaofoltran I've re-added CI so if you could rebase, that would be great.

[joaofoltran]

@mble rebased and some changes :)