Add support for DD_TAGS for AWS vpc flow logs metrics

[amitkc1]

Describe what happened:
We noticed that the lambda function for vpc flow logs metrics does not accept an environment variable (DD_TAGS) to pass addition tags. This can be useful to identify aws environment or vpc.

Describe what you expected:
In datadog, user should be able to identify/filter the vpc metrics using additional tags. These additional tags can be added using DD_TAGS in this lambda function.