[usage] Store credits as integer values in the usage table

[andrew-farries]

Description

Workspace credits per minute can be configured as fractional values, but we always want to bill and display usage in whole numbers of credits. This PR changes the type of the field that stores credits consumed per workspace instance to be an integer.

Related Issue(s)

Part of #9036 and this comment on #11406.

How to test

Connect to the database for the preview and describe the d_b_workspace_instance_usage table:

Release Notes

NONE

Documentation

Werft options:

• /werft with-preview

[werft-gitpod-dev-com]

started the job as gitpod-build-af-integer-credit-values-in-usage-store.2 because the annotations in the pull request description changed
(with .werft/ from main)

[geropl]

int(11) sounds potentially smaller than int64 which we use in code. Maybe a different length or bigint is more suitable? 🤔

[andrew-farries]

/werft run with-preview=true with-clean-slate-deployment=true

👍 started the job as gitpod-build-af-integer-credit-values-in-usage-store.6
(with .werft/ from main)

[andrew-farries]

Changed to bigint in 3c12d8c.

I think this is unnecessarily large though, but it's now consistent with the type used elsewhere.

[jldec]

My understanding was that we have to report total usage to Stripe as an int, but we should collect usage per session in fractional units to avoid rounding errors.

[easyCZ]

Indeed, the stored value should remain a float. This is because only the total billed value should be rounded, not each workspace instance.

[easyCZ]

Regular workspace costs 1 credit per 6 minutes, there's no way we'd be able to express workspace that ran for 5 minutes, or other fractions in this way.

[andrew-farries]

Indeed, the stored value should remain a float. This is because only the total billed value should be rounded, not each workspace instance.

In that case, I believe we are incorrectly rounding in the billing data sent to Stripe:

gitpod/components/usage/pkg/controller/reconciler.go

Lines 143 to 146 in a86efd2

	var credits int64
	for _, instance := range instances {
	credits += pricer.CreditsUsedByInstance(&instance, maxStopTime)
	}

Here we round each workspace instance's credits, rather than rounding the total once at the end.