Document the sdk/metric/aggregator/histogram package in the package godocs

[MrAlias]

Blocked: #2816

• Create sdk/metric/aggregator/histogram/doc.go with package documentation
  • Include documentation about what the package is used for
• Create sdk/metric/aggregator/histogram/example_test.go with example tests for the important parts of the package API.

[MrAlias]

The aggregations design no longer requires this.