[Metrics SDK] Boundaries type for Explicit Bucket Histogram Aggregation should be double

[lalitb]

Irrespective of the aggregation data type (long or double) for values stored in buckets, the boundaries range should be always of type double.

As mentioned in the specs - https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#explicit-bucket-histogram-aggregation