Add connection active, idle time to pg_stat_activity

Hello there hackers,

We at Zalando have faced some issues around long running idle
transactions and were thinking about increasing the visibility of
pg_stat_* views to capture them easily. What I found is that currently
in pg_stat_activity there is a lot of good information about the
current state of the process, but it is lacking the cumulative
information on how much time the connection spent being idle, idle in
transaction or active, we would like to see cumulative values for each
of these per connection. I believe it would be helpful for us and more
people out there if we could have total connection active and idle
time displayed in pg_stat_activity.

To provide this information I was digging into how the statistics
collector is working and found out there is already information like
total time that a connection is active as well as idle computed in
pgstat_report_activity[1]. Ideally, this would be the values we would
like to see per process in pg_stat_activity.

Curious to know your thoughts on this.

[1]https://github.com/postgres/postgres/blob/cd3f429d9565b2e5caf0980ea7c707e37bc3b317/src/backend/utils/activity/backend_status.c#L593

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Regards,
Rafia Sabih