[target-allocator] Apply relabel configs to targets prior to allocation

[kelseyma]

Can the target allocator apply the relabel configs prior to allocation so we can drop any unnecessary targets? This would also help reduce load on the collectors and ensure that the allocation is more evenly distributed.
For example in the test setup I have, the /metrics endpoint says the TA is allocating 36 targets, but only 3 of those targets are actually valid based on the relabel configs I set.

[jaronoff97]

Hey Kelsey! Thank you for submitting this issue, definitely something I've been thinking about too. My team can take a look at implementing this behavior. Not sure when we'll get to it, we may have some time in the coming weeks.