ZipkinHttpClientSender fails with "Failed to introspect Class" when spring-web is not on the classpath

[philwebb]

From @jonatan-ivanov:

I'm upgrading some of our (Micrometer) sample projects from Boot 3.2 to 3.3 and quite a few of out tests are failing because it seems ZipkinHttpClientSender depends on spring-web and the failing samples don't use web, see the import: org.springframework.http.HttpHeaders.

Maybe somehow it was on the classpath before but I think the Zipkin client should not depend on spring-web.

Caused by: java.lang.IllegalStateException: Failed to introspect Class [org.springframework.boot.actuate.autoconfigure.tracing.zipkin.ZipkinHttpClientSender] from ClassLoader [org.springframework.boot.loader.launch.LaunchedClassLoader@568db2f2]
Caused by: java.lang.NoClassDefFoundError: org/springframework/http/HttpHeaders    
Caused by: java.lang.ClassNotFoundException: org.springframework.http.HttpHeaders

[mhalbritter]

We need to do that in 3.3.x, too. I'm backporting the fix, have realized that too late.