fix(http): fall back to URL.Host for client server.address

[will-jg]

Summary

• net/http client instrumentation previously set server.address only from Request.Host.
• For client requests, Host is optional and may be empty while URL.Host is set; the transport still works, but metrics/spans got server.address="".
• Fall back to request.url.Host when Host is empty. GetServerPort is unchanged.

Test plan

• go test -run TestNetHttpClientGetServerAddress ./pkg/rules/http
• Confirm http.client.request.duration points no longer emit empty server.address for requests constructed with empty Request.Host and non-empty URL.Host