adding readable span

[codeboten]

Description

This PR adds ReadableSpan to provide read-only access to attributes of a Span

Fixes #1001

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

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

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

[toumorokoshi]

LGTM

[lzchen]

Can we lock down sampler, traceconfig and span_processor?

[lzchen]

As discussed with @codeboten , not certain yet if the current PR adds much value if we don't prevent the "writeable" property on the span when passed into the spanprocessor.

[codeboten]

As discussed with @codeboten , not certain yet if the current PR adds much value if we don't prevent the "writeable" property on the span when passed into the spanprocessor.

Span now returns a ReadableSpan on_end