add ability to observe deadline expiration counts

[bjlaub]

Provides the ability to observe the number of deadline expirations that happen between two points in time. Intended for consumers to be able to potentially build more customized observability outside of the metrics reported directly by this library.

A ExpirationObservation just reads the value of the internal meters between two points in time and exposes a count of the number of expirations that occurred.

Alternative to #92

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Type

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• Fix
• Break
• Deprecation
• Manual task
• Migration

Description

add ability to observe deadline expiration counts

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[bjlaub]

we could make this (or all of these) public to allow consumers to e.g. observe when a deadline expiration would throw an exception if enforcement is enabled.

[mpritham]

What are your thoughts on something like #95? I think it's a bit simpler: I'm proposing we just expose the deadline metric values and provide a small utility to compute diffs between the metric value observations.