Tasks:

• Rewrite IntegerHistogram so that it is just a generic Histogram.
• Add a template parameter to the class of the key type, rather than int64
  • Perhaps static_assert that the key type has operator< well-defined so that we can use std::upper_bound to find the insertion index
• Add a type alias for IntegerHistogram so that it has identical behavior to today. Should just be the now “generic” Histogram with int64_t as the first type parameter
• Add aliases for Duration types (like Milliseconds) to ensure they work.

Old text

Introduce a generic histogram type that can replace/extend IntegerHistogram and offer more flexibility. This new type should allow defining customized tags for each bucket when serializing the histogram, and support duration types (e.g., Milliseconds).