Introduce the new user-facing Logs API as a placeholder

[MSNev]

Part of open-telemetry/oteps#265

[trask]

(editorial suggestions only)

[tedsuo]

Looks great, let's get this merged and add the API method.

[MSNev]

@trask, @lmolkova, @pellared can you please check the latest commit, it (should) include

• all of your suggested changes (which I committed from github)
• As per discussion in the Maintainers meeting (general agreement of calling it the "Logs Instrumentation API"), updated the missed renames as per Trasks commits from last week.

Process taken: Merged changes from remote branch (which included the commits), updated missed renamings and pushed them back out (leading to loads of listed "commits" -- which should just be duplicative), I normally "cleanup" and squash the commits locally to keep it clean, but that requires a forced push to my branch and I didn't want to do that for this PR, hence the "messy" looking commits.

[trask]

I think we may want to just call it the "Logs API" (which is enough to differentiate it from the "Logs Bridge API"), but I can propose that in a follow-up.