Remove "best Histogram Aggregation available" option

[jack-berg]

The SDK histogram aggregation "informs the SDK to select the best Histogram Aggregation available". (Here's a conversation discussing the original inclusion of this language.)

If I understand correctly, this would reserve SDKs the right to change their definition of the "best histogram aggregation available" from one release to the next. For example, an SDK might choose to change the default bucket boundaries for the explicit bucket histogram, or an SDK might change from explicit bucket histogram to exponential histogram.

We discussed this in the Java SIG and believe that allowing for this type of change in behavior is unacceptable from a user experience standpoint. Changing either the bucket boundaries, or from explicit bucket to exponential histogram would change and potentially break backend behavior.

Instead, the "histogram" aggregation should have a consistent, reliable definition.