Emit HTTP engine thread count as a metric

[aajtodd]

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Describe the Feature

Add a new metric smithy.client.http.threads.count representing the current thread pool count (if applicable for an engine)

Is your Feature Request related to a problem?

Insight into the way an HTTP engine thread pool grows or shrinks w.r.t connections and load is a useful insight for tuning.

• I may be able to implement this feature request