refactor: uniquely identify {Value,Duration}Histograms by name and label sets

Previously, {Value,Duration}Histograms could only be identified by their name. This change allows multiple {Value,Duration}Histograms to share the same name, with each unique set of labels defining a distinct time series.

However, the buckets are immutable for a MetricGroup once initialized with the first metric.

Additionally, the internal data structure for a stored metric has been refactored, providing a more robust and programmatic representation.

Signed-off-by: Melissa Kilby <[email protected]>

Author: incertum