Add timer context manager for histogram

[srikanthccv]

Description

Fixes #2756

Type of change

Please delete options that are not relevant.

• [x] New feature (non-breaking change which adds functionality)

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

• [x] tox -e opentelemetry-sdk

Does This PR Require a Contrib Repo Change?

• [x] No.

Checklist:

• [x] Followed the style guidelines of this project
• [x] Changelogs have been updated
• [x] Unit tests have been added

[srikanthccv]

Meeting on 30th Jul: We won't make this part of the stable release. Probably create a convenience package.