Add an outcome tag to RestTemplate/WebClient metrics similar to that provided for WebMVC and WebFlux

[michaelmcfadyen]

Related to #13801

This effects spring boot actuator and specifically it's metrics classes.

Situation
I want to graph a count of calls made to a downstream api that returned a 4xx status code.

It would be useful to have a tag on those metrics that would group the response codes appropriately. For example these groupings could be:

• 1xx -> informational
• 2xx -> success
• 3xx -> redirection
• 4xx -> client_error
• 5xx -> server_error

This tag has already been added to the web mvc metrics.

[wilkinsona]

Sounds sensible to me, thanks for the suggestion. In the meantime, you can implement this yourself by configuring a custom RestTemplateExchangeTagsProvider bean.

[snicoll]

We should make sure this gets added to WebClient as well for consistency.