Add option to send additional metric tags for Open Metrics

[ofek]

Motivation

Often need to send more than user-defined custom_tags

[nmuesch]

This looks good to me, though can we get a review from @DataDog/container-integrations as well?

[mfpierre]

I'm not sure I get the difference between custom_tags and _metric_tags, what's the need?

Is this setting used in some other checks?

[zippolyte]

custom_tags are user defined tags in the configuration file of the check. They should not be modified by the check to pass in metric tags, hence the need of an additional field.
This is particularly important in the case of the etcd check (v3 uses openmetrics), because we tag service checks with custom tags defined by the user, and we don't want the metric tags to be associated with them.