fix: rows synced doesn't exist

[raquelmsmith]

Problem

I noticed a few days ago (and then forgot 😬 ) that we are getting a ton of errors for missing rows_synced key. https://posthog.sentry.io/issues/4640389037/?project=1899813&query=is:unresolved+!url:http://billing.dev.posthog.dev+usage&statsPeriod=14d&stream_index=0

Then today Cameron found this, which led me down a rabbit hole to realize that orgs aren't getting properly quota limited. Someone's billing limit will be set, but they are still accruing usage (and not being charged for it, thankfully).

I'm wondering if this is the culprit. I don't really know if my fix here is the right solution... does the key not exist in the usage record because it doesn't exist in billing?

Changes

.get()s the key instead of assuming it's there.

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How did you test this code?

[EDsCODE]

1. If we want the safest option, we could just remove "rows_synced" for now until I loop back ready for billing
2. I think it would make more sense if we returned the default payloads that would indicate 0 usage. So for resource_usage = new_usage.get(field, {<default usage schema>}) and resource_usage["todays_usage"] = todays_usage.get(field, 0)

[raquelmsmith]

"rows_synced" is kinda all over the place, we'll try the default value, that's a good idea.