update: apply reviewers suggestions

Reduce code duplication further by adding `removing`, `getMetric` and `forEachMetric` to `MetricFamily`.
Rename `add` -> `adding` to indicate non mutating nature (Swift convention).

Co-authored-by: Konrad `ktoso` Malawski <[email protected]>
Signed-off-by: Melissa Kilby <[email protected]>

Author: incertum