opentelemetry-instrumentation: add unwrapping from dotted paths strings

[xrmx]

Description

Make it possible to express object to unwrap as dotted module paths strings. This helps in avoiding side effects or race conditions with other instrumentations if we do importing too early.

In practice this permits to call unwrap like:

unwrap("mymodule.myclass", "my_method")

to avoid any eventual side effect that import mymodule at the head of the instrumentator file may have.

It also matches wrap_function_wrapper being able to import from dotted string.

While at it add tests also for current functionality.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• tox

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated