code: @PostMapping(path = "/instances", consumes = MediaType.APPLICATION_JSON_VALUE)
      public Mono<ResponseEntity<Map<String, InstanceId>>> register(@RequestBody Registration registration,
                                                                    UriComponentsBuilder builder) {
          Registration withSource = Registration.copyOf(registration).source("http-api").build();
          LOGGER.debug("Register instance {}", withSource);
          return registry.register(withSource).map(id -> {
              URI location = builder.replacePath("/instances/{id}").buildAndExpand(id).toUri();
              return ResponseEntity.created(location).body(Collections.singletonMap("id", id));
          });
      }
docs: Register an instance.
  
  @param registration registration info
  @param builder      UriComponentsBuilder
  @return The registered instance id;