add default values for sums of apiserver_request:availability30d

[Mx7ca]

This PR should fix #1037 and in the end also fix prometheus-community/helm-charts#5407

More information can be found in prometheus-community/helm-charts#5415 where the change has been explained in more detail

[lorenzofelletti]

Hi! I see what you're trying to achieve with this PR, and I agree with you that fallbacks value vector(0) should be provided in case there are no slow writes, etc.

However, sometimes no data is the actually correct answer, when the value for the expression is actually impossible to calculate.

Thus, I would advise for some default values you added in this PR to be removed.

[Mx7ca]

@lorenzofelletti I've removed some of the defaults again. Especially when something was subtracted, I've made sure that the subtrahend will not be calculated before the minuend (see subtraction-terms) because otherwise it could result in

0 (default) - X

and therefore result in a misleading difference.

Could you please check it out again?

[lorenzofelletti]

Thank you @Mx7ca ! I think this is a very valuable contribution!

[skl]

Thanks all for review and discussion