Lazy load boto client when using datadogpy for metrics.

[purple4reina]

What does this PR do?

Lazy load boto3 and botocore libraries to use only when required.

Motivation

Faster cold starts when using datadogpy for sending metrics.

Testing Guidelines

Additional Notes

Original method for creating boto3 client

import boto3
boto3.client('kms')

But boto3.client code is just (see https://github.com/boto/boto3/blob/2b18684afe102cfd28f9ddc873d7862209d1da69/boto3/__init__.py#L92)

return _get_default_session().client(*args, **kwargs)

where _get_default_session() is defined as (see https://github.com/boto/boto3/blob/2b18684afe102cfd28f9ddc873d7862209d1da69/boto3/__init__.py#L34)

from boto3.session import Session
DEFAULT_SESSION = Session(**kwargs)

The client method on a Session is just (see https://github.com/boto/boto3/blob/2b18684afe102cfd28f9ddc873d7862209d1da69/boto3/session.py#L215)

return self._session.create_client(*args, **kwargs)

Where self._session is an instance of botocore.session.Session (using botocore.session.get_session see https://github.com/boto/boto3/blob/2b18684afe102cfd28f9ddc873d7862209d1da69/boto3/session.py#L62)

Long story short, we can avoid importing all of boto3!!

Types of Changes

• Bug fix
• 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)