Add Logging Library SDK Prototype Specification

[tigrannajaryan]

This is a draft proposal for OpenTelemetry Logging Library SDK specification.
The purpose of his proposal is to lay out the initial understanding of
what Logging Library SDKs will look like.

We would like to create prototypes based on this proposal in a few languages.
The learnings from prototypes will then allow us to refine this proposal and
eventually make the proposed approach part of the OpenTelemetry specification.

The approach proposed in this OTEP is not intended to be merged into the
OpenTelemetry specification after the OTEP itself is approved. We want to have
successful prototypes first. This OTEP is first and foremost a specification and
guidelines on how to create the prototypes.

The specification defines how the OpenTelemetry Logging Library SDK exposes its
functionality to authors of extensions to language-specific 3rd party logging
libraries and to end users that want to produce logs in
OpenTelemetry manner.

The specification defines SDK elements that to some extent mirror the
OpenTelemetry
Trace SDK.
This ensures uniformity and consistency of the OpenTelemetry specification and
of the implementations across traces and logs. For additional clarity the
definitions in this document refer to the Trace analogs where appropriate.

The descriptions of interfaces and methods in this document are intentionally
very brief. Detailed and precise descriptions will be borrowed from Trace
specification when this OTEP is submitted as a Log specification PR. I kept the
descriptions as short as possible in this document to make reviewing it easy and
fast.

[tigrannajaryan]

@pmm-sumo for some reason I cannot add you as a reviewer, please review.

[tigrannajaryan]

Note: I am out tomorrow, won't be able to respond to comments until the day after.

[bogdandrutu]

For a prototype phase these specifications are good

[tigrannajaryan]

@open-telemetry/python-approvers I think there was some recent log-related work done in Python library. Please review this proposal.

[tigrannajaryan]

@open-telemetry/cpp-approvers I believe we have a Log prototype in C++ library. Please review this proposal and see if it fits what you wanted to do in C++.

[jkwatson]

Looks like a good starting point to me.

[lalitb]

LGTM in general. Good start for initial prototyping.

[owais]

LGTM

[tigrannajaryan]

Has the necessary number of approvals. No new comments for 3 days. All comments resolved. Merging.

If anyone sees this PR after merging and has thoughts please do speak: we want to know your opinion and can adjust the OTEP and our prototyping approach.