Add document for gPRC metrics with armeria-spring-boot (#4296)
Add document for gPRC metrics with armeria-spring-boot (#4296)

Motivation:

This explanation is useful for users who want to collect gRPC metrics with spring-boot-armeria

Result:

- Closes #4197 
trustin/armeria,minwoox/armeria,line/armeria,trustin/armeria,line/armeria,kojilin/armeria,kojilin/armeria,minwoox/armeria,minwoox/armeria,kojilin/armeria,trustin/armeria,minwoox/armeria,kojilin/armeria,minwoox/armeria,kojilin/armeria,line/armeria,kojilin/armeria,trustin/armeria,trustin/armeria,line/armeria,trustin/armeria,line/armeria,line/armeria,minwoox/armeria