Proxy tracer/provider to enable lazy setup of tracing pipeline

[owais]

Description

Introduces a proxy tracer provider and proxy tracer implementations. The proxies return noop implementations until a global tracer provider is set by the user. Once a global provider is set the proxies delegate to the real provider and tracer implementations.

Updated get_tracer_provider() to return a proxy tracer provider if a the user has not set a global provider via code or env vars.

Fixes #1159
Fixes #1276
Fixes open-telemetry/opentelemetry-python-contrib#405

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?

• Added new test case
• Manually tested with https://github.com/owais/otel-django-test

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR: Account for ProxyTracerProvider opentelemetry-python-contrib#399
• No.

Checklist:

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

[owais]

Docker hub is having issues right now and as a result docker test can't pull some images. Either that or the images were deleted. Will take a look tomorrow if it doesn't fix on it's own.