labels.Matcher: Improve regexp and allow escaping
labels.Matcher: Improve regexp and allow escaping

This addresses a number of issues:

- It was impossible to include a literal '"' or a line break in the value.
- It was impossible to include '=', '~', or '!' in the value.
- It was not validated if the label name is valid.
- It was not validated if the value is valid UTF-8.
- No whitespace was allowed around the operator.

Signed-off-by: beorn7 <91e32ae1d26bd312b370cf8378a5a83accca90a0@grafana.com>

prometheus/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager,prometheus/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager,GoogleCloudPlatform/alertmanager,GoogleCloudPlatform/alertmanager