opentelemetry: improve the 0.12 changelog (#436)
opentelemetry: improve the 0.12 changelog (#436)

The api module (not a crate) was already private, so that entry doesn't seem to be useful
for most users. Additionally, this missed the removal of the http-related types and
implementations, which probably is relevant to a larger audience.
open-telemetry/opentelemetry-rust,open-telemetry/opentelemetry-rust