Scott.solmonson/batch item failure metric

[ssolmonson]

What does this PR do?

Creates a metric for batch item failures and closes issue #435

Motivation

435

Testing Guidelines

• Added unit tests
• Tested the changes manually

Additional Notes

SVLS-3126

Types of Changes

• [ ] Bug fix
• [x] New feature
• [ ] Breaking change
• [ ] Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• [ ] This PR's description is comprehensive
• [ ] This PR contains breaking changes that are documented in the description
• [ ] This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• [ ] This PR impacts documentation, and it has been updated (or a ticket has been logged)
• [ ] This PR's changes are covered by the automated tests
• [ ] This PR collects user input/sensitive content into Datadog
• [ ] This PR passes the integration tests (ask a Datadog member to run the tests)