allow customizing of retry logging handler

[alexkli]

Current Behaviour

Currently node-fetch-retry will console.log or console.error on retry (and other cases), with a specific message. For example here.

Inventory of console.*:

on retry:

• https://github.com/adobe/node-fetch-retry/blob/master/index.js#L158 (shouldRetryOnHttpError a)
• https://github.com/adobe/node-fetch-retry/blob/master/index.js#L161 (shouldRetryOnHttpError b)
• https://github.com/adobe/node-fetch-retry/blob/master/index.js#L221 (on http response not ok)
• https://github.com/adobe/node-fetch-retry/blob/master/index.js#L235 (on error thrown)

on init (config read):

• https://github.com/adobe/node-fetch-retry/blob/master/index.js#L90 (on forceSocketTimeout)

Issue

Apps using node-fetch-try might want a different log message or use a different logging.

For example, they might want to log error details from a specific header or the response body. Currently it only prints the response.statusText in case of a non-success HTTP response.

Apps should be in control and be able to have node-fetch-retry not call console.* at all.

Proposal

Add the option for a custom function onRetry in the retryOptions that would be used instead of the current logging. Signature might follow the usual pattern

function onRetry(response, error) {
   // response if response was returned otherwise null
   // error if an error was thrown
}

Note sure how to handle the "on init" case, maybe another onInitLog function. Or use the npm debug library (which is off by default).