Add http.route to HTTP client requests and extract from Spring WebClient

[HaloFour]

Adds the semantic attribute http.route to the list of attributes supported by HTTP client requests, indicating a templated URI used when making the HTTP request as supported by the client framework.

Adds support for extracting that templated URI from the ClientRequest attributes when using Spring WebClient. This is based on the Spring actuator implementation for Spring Boot 2.x that extracts the URI template and formats it for the purposes of reporting to Micrometer:

https://github.com/spring-projects/spring-boot/blob/2.7.x/spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/web/reactive/client/WebClientExchangeTags.java#L68

This implementation returns null if the attribute is not found to avoid reporting raw URIs.

[missTam]

Hi @HaloFour @trask,

Will this PR be revisited?

Is there a current practical use of #675 without the underlying HTTP client infrastructure supporting it?

Is this the required implementation for it?

Please check the discussion.

Thank you!