chore: Update prometheus to version v3.

[simonswine]

Note this is change of behaviour as it will relax the label validation and allow the newer metric selectors format, which in turn needs to be used to use the full character set.

There is also some bigger impact to the remote write/metrics export, as there is a new version of remote write in prometheus v3: https://prometheus.io/docs/specs/prw/remote_write_spec_2_0/

This will be the new label selector format:

{"metric.name", "my.label.name"="bar"}

In order to override other validation at a later point, we also need to set:

import "github.com/prometheus/common/model"

func init() {
    model.NameValidationScheme = model.UTF8Validation
}

I am not planning to do this as part of this PR.

[marcsanmi]

LGTM 👍

We need to update prometheus testutils that has changed and some tests that now with a more relaxed label validation might expect or fail with different results.