Implement Micrometer TCK for RestTemplate and WebClient metrics

[mhalbritter]

Micrometer has added a HTTP client TCK.

To quote @shakuzen:

The idea of the TCK was just to cover common concepts across projects so we could ensure that regardless of what http client implementation you are using you can expect a common subset of metrics from them all.

[...] for now Boot can implement the http client metrics TCK for RestTemplate and WebClient in 2.x. Please share any feedback on the usability of the TCK or improvements to the testing there. We'll later add an http server metrics TCK to Micrometer.