Add retries metrics when used with openwhisk

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?

Start by reading the retry request handling in node-fetch-retry and review how openwhisk-newrelic consumes x-http-metrics-* headers. Clarify the header names, whether the final-retry state is knowable, and whether sending these headers is acceptable for other users. Done means the retry count and final-retry status are consistently exposed without unintended request headers.

Written by the indexing model from the issue text.