add buffered_metrics object type

[andrewqian2001datadog]

What does this PR do?

Adds BufferedMetric object to allow for extended aggregation (Distribution, Histogram, Timing metrics)

Based on off existing Go code

Description of the Change

Adds a buffered_metrics.py class that has all the properties that Distribution, Histogram and Timing metrics require

Verification Process

Added unit tests matching the existing tests

Additional Notes

1. This PR will merge onto a feature branch.

Review checklist (to be filled by reviewers)

• Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have one changelog/ label attached. If applicable it should have the backward-incompatible label attached.
• PR should not have do-not-merge/ label attached.
• If Applicable, issue must have kind/ and severity/ labels attached at least.