Rename Record class in Metrics SDK to Accumulation to follow spec

[AzfaarQureshi]

Description

This PR addresses Issue open-telemetry#1343 and issue open-telemetry#1307 by renaming the Record class in the metrics sdk to Accumulation

related to PR #6 and issue open-telemetry#1342

Summary of Changes

1. rename Record class to Accumulation
2. rename all record = metrics.Accumulation() to accumulation = metrics.Accumulation() for consistency

Rationale behind changes

1. rename Record class to Accumulation

• PR Renaming Meter to Accumulator in Metrics SDK context #6 and issue Refactor Python Metrics SDK to use naming consistent with Metrics Specification  open-telemetry/opentelemetry-python#1307 showed that the Meter class should be renamed to Accumulator
• According to the Spec, the type computed/collected by the Accumulator must be named Accumulation

2. rename all record = metrics.Accumulation() to accumulation = metrics.Accumulation() for consistency

• cleaning up vestigial naming scheme from when Accumulation used to be called Record.
• variable name should reflect what is actually behind returned

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?

Just made sure all the tests are passing post naming change

Checklist:

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

[alolita]

lgtm