Ensure burn rate factors are accurate, and add exhaustion label

[saswatamcode]

This PR does the following,

• Updates the burn rate factors in Windows() function from 14, 7, 2, 1 to 14, 5.6, 2.8, 1 to more accurately represent 28 day burn rate in the following cases (as per conversation offline with @metalmatze),
  • 50% error budget depleted in a day / 100% in 2 days
  • 20% error budget depleted in a day / 100% in 5 days
  • 10% error budget depleted in a day / 100% in 10 days
  • 100% error budget depleted in 28 days
• It also adds goDoc comments to Windows which describe its functionality for posterity.
• Adds a new exhaustion label to all budget burn alerts, which represents time till the error budget is fully exhausted.
• Adds new test cases to Test_windows to account for various common SLO windows like 90d, 84d, 30d, 28d, 14d & 7d

Most of this large diff is to account for changes in tests. Let me know what you think! 🙂

[moadz]

LGTM :)

[metalmatze]

Super cool! Thanks.
Especially adding more tests all over the place! 👍

I'm still not 100% sure about the exhaustion label, since the budget might be gone way quicker depending on the current budget that's left and the potentially uneven amount of events happening over time. For now, let's see how it goes with that label.

[metalmatze]

Long time no see.
Would you still like to get this in?