Add metric_names_limit to scrape config

[SuperQ]

Proposal

In order to stop targets from overloading the __name__ index, add a scrape config limiter that limits the number of metric names per scrape.

This will protect, for example, when other metric systems are translated to Prometheus incorrectly and tag values become part of the metric name.

[roidelapluie]

Should we enable that setting for any label name?

[SuperQ]

That's not a bad idea. Would we want one setting for metric names and one for label values?

label_cardinality_limit

[roidelapluie]

The metric name is a label value.