feat(metrics/family): add get method to Family

[ethercflow]

Address comments in #230 (comment)

[ethercflow]

@mxinden PTAL, thanks!

[cratelyn]

seconding this addition, i would also suppport this change, and would like have the ability to avoid creating new metrics in a family when looking up the series for a particular label set. i'll cross-reference #244, which also introduces a new accessor to Family<S, M, C>.

[mxinden]

Thank you for the thorough patch.

What do you think of replacing the code below in Family::get_or_create with a call to Family::get?

        if let Ok(metric) =
            RwLockReadGuard::try_map(self.metrics.read(), |metrics| metrics.get(label_set))
        {
            return metric;
        }

Also, please add a changelog entry.

[ethercflow]

@mxinden Addressed, PTAL, thanks!

[mxinden]

Thank you!