Fixes#13430.
The test for purging old workspace agent stats from the DB was consistently failing when ran with Postgres towards the end of the month, but not with the in-memory DB.
This was because month intervals are calculated differently for `time.Time` and the `interval` type in Postgres:
```
ethan=# SELECT
'2024-08-30'::DATE AS original_date,
('2024-08-30'::DATE - INTERVAL '6 months') AS sub_date;
original_date | sub_date
---------------+---------------------
2024-08-30 | 2024-02-29 00:00:00
(1 row)
```
Using `func (t Time) AddDate(years int, months int, days int) Time`, where `months` is `-6`:
```
Original: 2024-08-30 00:00:00 +0000 UTC
6 Months Earlier: 2024-03-01 00:00:00 +0000 UTC
```
Since 6 months was chosen arbitrarily, we should be able to change it to 180 days, to remove any ambiguity between the in-memory DB, and the Postgres DB. The alternative solution would involve implementing Postgres' month interval algorithm in Go.
The UI only shows stats as old as 168 days (24 weeks), so a frontend change isn't required for the extra days of stats we lose in some cases.
* chore: remove InsertWorkspaceAgentStat query
InsertWorkspaceAgentStats (batch) exists. We only used the singular in
a single unit test place. Removing the single for the batch, reducing
the interface size.
* fix: don't make session counts cumulative
This made for some weird tracking... we want the point-in-time
number of counts!
* Add databasefake query for getting agent stats
* Add deployment stats endpoint
* The query... works?!?
* Fix aggregation query
* Select from multiple tables instead
* Fix continuous stats
* Increase period of stat refreshes
* Add workspace counts to deployment stats
* fmt
* Add a slight bit of responsiveness
* Fix template version editor overflow
* Add refresh button
* Fix font family on button
* Fix latest stat being reported
* Revert agent conn stats
* Fix linting error
* Fix tests
* Fix gen
* Fix migrations
* Block on sending stat updates
* Add test fixtures
* Fix response structure
* make gen
* chore: convert workspace agent stats from json to table
* chore: convert agent stats to use a table
Backwards compatibility becomes hard when all agent stats are in a JSON blob.
We also want to query this table for new agents that are failing health checks
so we can display it in the UI.
* Fix migration using default values
