[feature] Add healthcheck endpoints /livez and /readyz

[tsmethurst]

Description

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This pull request adds some simple health check endpoints, and documentation for how to configure a health check.

Closes #2707

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[NyaaaWhatsUpDoc]

@tsmethurst this otherwise looks good and ready to go, but just one question: would it be worth instead doing a database ping instead of an actual select on a table? or instead just do a select on a table with no columns (just so we don't end up doing any actual searching).

[tsmethurst]

Mm, the nice thing about doing a little select instead of a ping is that I don't think pinging catches things like database locked etc. Actually selecting something is a bit more of a representative test of "is this thing on", imo.

or instead just do a select on a table with no columns (just so we don't end up doing any actual searching).

I'd be happy to change it to that yeah, like just a SELECT * [...] LIMIT 1 you mean? Is that cheaper?

[NyaaaWhatsUpDoc]

Mm, the nice thing about doing a little select instead of a ping is that I don't think pinging catches things like database locked etc. Actually selecting something is a bit more of a representative test of "is this thing on", imo.

or instead just do a select on a table with no columns (just so we don't end up doing any actual searching).

I'd be happy to change it to that yeah, like just a SELECT * [...] LIMIT 1 you mean? Is that cheaper?

no i mean you can write a statement that basically returns just none of the columns, so for example here's:

sqlite> select null from instances limit 0;
Run Time: real 0.000 user 0.000075 sys 0.000000

and then compared to the original:

sqlite> select id from instances where domain = "k.iim.gay";
01A0XSYN1D4BG83VMGPEV17PH2
Run Time: real 0.000 user 0.000177 sys 0.000000

not a huge difference given they're both absurdly fast, but there is some

EDIT:
if you set .timer onwhen in sqlite interactive you can play around a bit and see. obviously there's some variance between each time you execute them but it's generally useful for showing ballpark figures if you run it a few times

[tsmethurst]

Got it! Changed it here 4924713 and it seems to work for both sqlite and postgres :)