springboot URI tag for WebMvc and WebFlux metrics is empty, rather than "root", when the path pattern is empty

This handles the case where you have an empty path in @GetMapping, which yields an empty string for the uri tag.

# HELP http_server_requests_seconds  
# TYPE http_server_requests_seconds summary
http_server_requests_seconds_count{exception="None",method="GET",outcome="SUCCESS",status="200",uri="/actuator/prometheus",} 48764.0
http_server_requests_seconds_sum{exception="None",method="GET",outcome="SUCCESS",status="200",uri="/actuator/prometheus",} 1460.28618
http_server_requests_seconds_count{exception="None",method="GET",outcome="SUCCESS",status="200",uri="",} 1.0
http_server_requests_seconds_sum{exception="None",method="GET",outcome="SUCCESS",status="200",uri="",} 0.055319
# HELP http_server_requests_seconds_max  
# TYPE http_server_requests_seconds_max gauge
http_server_requests_seconds_max{exception="None",method="GET",outcome="SUCCESS",status="200",uri="/actuator/prometheus",} 0.0599019
http_server_requests_seconds_max{exception="None",method="GET",outcome="SUCCESS",status="200",uri="",} 0.055319

Comment From: snicoll

Thanks @jkschneider