Fix duplicate enhanced metrics

[tianchu]

What does this PR do?

If the datadog serverless plugin is used, which automatically wraps the function's handler with @datadog_lambda_wrapper, and the handler function is also decorated with @datadog_lambda_wrapper, the resulted code is equivalent to the following, and two wrappers get created.

@datadog_lambda_wrapper
def lambda_handler(event, context):
    pass

def plugin_generated_lambda_handler(event, context):
    datadog_lambda_wrapper(lambda_handler)(event, context)

This results in __init__ get called twice on cold start and __call__ gets called twice on every invocation. Because not every operation in those two methods are idempotent, this causes unexpected behaviors. E.g., aws.lambda.enhanced.invocations will get submitted twice for every single invocation.

Let @datadog_lambda_wrapper instantiate a no-op decorator _NoopDecorator when a wrapper is already instantiated. But allow _force_new to force create a new/real wrapper every time, which is needed by the unit tests.

[sfirrin]

looks good

[sfirrin]

nice idea