Stop using KV as the Key for Histogram Metrics

[JayajP]

Currenlty, PerWorkerHistograms are in a Map that's keyed by KV<MetricName, BucketType>. After this change the map they're stored in will be Keyed by MetricName.

Computing a hash/equality for the KV object is non-trivial and has a fair amount of overhead src. Since we need to do a map lookup for the histogram metric every time we record a value, we can remove a lot of the overhead by simply using a map that's only Keyed by MetricName.

The first time a Histogram is requested for a <Metric Name, Bucket Type> we will use the BucketType to create the histogram object. Future requests for a Histogram for that Metric Name will return the original histogram object, regardless of what the future Bucket Type is. Since Bucket Types are statically defined, this change does not affect BigQuery Per Worker Metrics.

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[JayajP]

R: @m-trieu

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