This commit extends Prometheus with the option to run aggregation rules
during scrape time, before relabeling takes place.
All rules are executed independently for each target in the context of
individual scrapes. Newly calculated samples are
committed to TSDB atomically with all samples from that scrape.
Metric relabeling can also be applied on series recorded from scrape rules.
Signed-off-by: Filip Petkovski <filip.petkovsky@gmail.com>
