* scrape: add label limits per scrape

Add three new limits to the scrape configuration to provide some
mechanism to defend against unbound number of labels and excessive
label lengths. If any of these limits are broken by a sample from a
scrape, the whole scrape will fail. For all of these configuration
options, a zero value means no limit.

The `label_limit` configuration will provide a mechanism to bound the
number of labels per-scrape of a certain sample to a user defined limit.
This limit will be tested against the sample labels plus the discovery
labels, but it will exclude the __name__ from the count since it is a
mandatory Prometheus label to which applying constraints isn't
meaningful.

The `label_name_length_limit` and `label_value_length_limit` will
prevent having labels of excessive lengths. These limits also skip the
__name__ label for the same reasons as the `label_limit` option and will
also make the scrape fail if any sample has a label name/value length
that exceed the predefined limits.

Signed-off-by: Damien Grisonnet <dgrisonn@redhat.com>

* scrape: add metrics and alert to label limits

Add three gauge, one for each label limit to easily access the
limit set by a certain scrape target.
Also add a counter to count the number of targets that exceeded the
label limits and thus were dropped. This is useful for the
`PrometheusLabelLimitHit` alert that will notify the users that scraping
some targets failed because they had samples exceeding the label limits
defined in the scrape configuration.

Signed-off-by: Damien Grisonnet <dgrisonn@redhat.com>

* scrape: apply label limits to __name__ label

Apply limits to the __name__ label that was previously skipped and
truncate the label names and values in the error messages as they can be
very very long.

Signed-off-by: Damien Grisonnet <dgrisonn@redhat.com>

* scrape: remove label limits gauges and refactor

Remove `prometheus_target_scrape_pool_label_limit`,
`prometheus_target_scrape_pool_label_name_length_limit`, and
`prometheus_target_scrape_pool_label_value_length_limit` as they are not
really useful since we don't have the information on the labels in it.

Signed-off-by: Damien Grisonnet <dgrisonn@redhat.com>