flush() doesn't clear properties

According the README, `flush()` "[f]lushes the current MetricsContext to the configured sink and resets all properties, dimensions and metric values. The namespace and default dimensions will be preserved across flushes."

However, in `MetricContext.create_copy_with_context()`, which is used by `MetricsLogger.flush()` to reset the context, [copies the old properties](https://github.com/awslabs/aws-embedded-metrics-python/blob/c85dcdd9a45be18aba7ffbcbd0e540a7a88d55f7/aws_embedded_metrics/logger/metrics_context.py#L127). This means in my Lambda function, if I have conditional properties in my logging object, once I've set one in one invocation, it remains present in the context in future invocations.