Define a way to indicate non-end-user available symbols and private class symbols

[ocelotl]

    It looks like the `_ExponentialBucketHistogramAggregation` is reading/setting these fields. Since it's already in `_internal`, I think we can should make these public properties to make it clear that these are being modified from outside of this class

Originally posted by @aabmass in https://github.com/open-telemetry/opentelemetry-python/pull/2964#discussion_r1049979361