Add retries metrics when used with openwhisk

[tmathern]

To track retries in http metrics:

• node-fetch-retry sets a special request header (name to be clarified) with the current retry count
• node-fetch-retry sets a special request header (name to be clarified) if it's the last retry (can we know this when we make the requests?)
• openwhisk-newrelic takes any "x-http-metrics-: " header and adds it as = attribute to the http metrics, then removes the header so that it does not actually get sent with the request
• not everybody using node-fetch-retry uses openwhisk-newrelic, so is it ok to send those headers in this case?