Add interface schema and docs for otlp interface

We landed the `otlp` interface library in https://github.com/canonical/charmlibs/pull/352 without interface docs and schema in the currently standard format (they're folded into the library instead). This should be addressed in a follow-up PR.

We're planning some changes around the standard format, so let's tackle this afterwards as an early adopter of the new approach.

Read the otlp interface library and pull request #352, then review the planned changes to the standard format before starting. Add the interface schema and documentation in the new standard format rather than keeping them folded into the library; done means both are provided separately.

Written by the indexing model from the issue text.