fix: fail health checks for inconsistent metrics

[immanuwell]

Fixes #544

Right now /-/healthy can still say 200 OK when /metrics is already busted after unchecked inconsistent pushes. This is easy to hit in practice, it only takes two tiny pushes when --push.disable-consistency-check is on.

This patch makes the health check fail when the gathered metrics are inconsistent, so k8s and friends can see the box is not ok.

Repro:

go run . --push.disable-consistency-check --web.listen-address=127.0.0.1:19091
curl -X POST --data-binary $'# TYPE some_metric counter\nsome_metric 1\n' http://127.0.0.1:19091/metrics/job/some_job/tag/val1
curl -X POST --data-binary $'# TYPE some_metric counter\nsome_metric{tag="val1"} 42\n' http://127.0.0.1:19091/metrics/job/some_job
curl -i http://127.0.0.1:19091/metrics
curl -i http://127.0.0.1:19091/-/healthy

Before this change /metrics returns 500 but /-/healthy still returns 200. After this change both return 500, nice and simple.

cc @machine424